bug#67848: 29.1; Fix usage of `setq-default' and add explanation

[Xiyue Deng]

In the lisp intro book, there is a discrepancy in the section "Text and
Auto Fill mode" where the first code example uses "(setq-default
major-mode 'text-mode)"[1] but in the later explanation part the
function was changed to `setq'[2].  It looks like the original change[3]
was to deprecate the usage of "default-major-mode", but forgot to update
the other part.

I have prepared a patch to make it consistent, and also added some extra
text to explain why `setq-default' is required for a buffer local
variable using a foot note.  Will add in the next mail.

[1] 
https://git.savannah.gnu.org/cgit/emacs.git/tree/doc/lispintro/emacs-lisp-intro.texi#n16902
[2] 
https://git.savannah.gnu.org/cgit/emacs.git/tree/doc/lispintro/emacs-lisp-intro.texi#n16946
[3] 
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=cd61af01c1220b6a7f98189c826edc7535416583



In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.37,
 cairo version 1.16.0) of 2023-09-19, modified by Debian built on
 debian-hx90
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --with-cairo --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -ffile-prefix-map=/build/emacs-bYKTEl/emacs-29.1+1=. -fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Texinfo/P

Minor modes in effect:
  TeX-PDF-mode: t
  global-git-commit-mode: t
  magit-auto-revert-mode: t
  shell-dirtrack-mode: t
  windmove-mode: t
  flyspell-mode: t
  rcirc-track-minor-mode: t
  server-mode: t
  bug-reference-prog-mode: t
  global-company-mode: t
  company-mode: t
  global-treesit-auto-mode: t
  icomplete-mode: t
  fido-mode: t
  override-global-mode: t
  global-display-line-numbers-mode: t
  display-line-numbers-mode: t
  global-auto-revert-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/debian-el-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/debian-el-autoloads
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/apt-sources hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/apt-sources
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/debian-bug hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/debian-bug
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/apt-utils hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/apt-utils
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/debian-el-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/debian-el-pkg
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/gnus-BTS hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/gnus-BTS
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/deb-view hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/deb-view
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/debian-el hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/debian-el
/usr/share/emacs/site-lisp/elpa/debian-el-37.11/preseed hides 
/usr/share/emacs/site-lisp/elpa-src/debian-el-37.11/preseed
/usr/share/emacs/site-lisp/elpa/devscripts-40/devscripts hides 
/usr/share/emacs/site-lisp/elpa-src/devscripts-40/devscripts
/usr/share/emacs/site-lisp/elpa/devscripts-40/devscripts-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/devscripts-40/devscripts-autoloads
/usr/share/emacs/site-lisp/elpa/devscripts-40/pbuilder-mode hides 
/usr/share/emacs/site-lisp/elpa-src/devscripts-40/pbuilder-mode
/usr/share/emacs/site-lisp/elpa/devscripts-40/devscripts-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/devscripts-40/devscripts-pkg
/usr/share/emacs/site-lisp/elpa/devscripts-40/pbuilder-log-view-mode hides 
/usr/share/emacs/site-lisp/elpa-src/devscripts-40/pbuilder-log-view-mode
/usr/share/emacs/site-lisp/elpa/dockerfile-mode-1.7/dockerfile-mode hides 
/usr/share/emacs/site-lisp/elpa-src/dockerfile-mode-1.7/dockerfile-mode
/usr/share/emacs/site-lisp/elpa/dockerfile-mode-1.7/dockerfile-mode-autoloads 
hides 
/usr/share/emacs/site-lisp/elpa-src/dockerfile-mode-1.7/dockerfile-mode-autoloads
/usr/share/emacs/site-lisp/elpa/dockerfile-mode-1.7/dockerfile-mode-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/dockerfile-mode-1.7/dockerfile-mode-pkg
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/debian-bts-control hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/debian-bts-control
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/debian-changelog-mode hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/debian-changelog-mode
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/dpkg-dev-el-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/dpkg-dev-el-autoloads
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/dpkg-dev-el-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/dpkg-dev-el-pkg
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/dpkg-dev-el hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/dpkg-dev-el
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/debian-control-mode hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/debian-control-mode
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/debian-copyright hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/debian-copyright
/usr/share/emacs/site-lisp/elpa/dpkg-dev-el-37.10/readme-debian hides 
/usr/share/emacs/site-lisp/elpa-src/dpkg-dev-el-37.10/readme-debian
/usr/share/emacs/site-lisp/elpa/lintian-0.1/lintian-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/lintian-0.1/lintian-pkg
/usr/share/emacs/site-lisp/elpa/lintian-0.1/lintian-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/lintian-0.1/lintian-autoloads
/usr/share/emacs/site-lisp/elpa/lintian-0.1/lintian hides 
/usr/share/emacs/site-lisp/elpa-src/lintian-0.1/lintian
/usr/share/emacs/site-lisp/elpa/po-mode-0.21/po-mode-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/po-mode-0.21/po-mode-pkg
/usr/share/emacs/site-lisp/elpa/po-mode-0.21/po-mode-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/po-mode-0.21/po-mode-autoloads
/usr/share/emacs/site-lisp/elpa/po-mode-0.21/po-mode hides 
/usr/share/emacs/site-lisp/elpa-src/po-mode-0.21/po-mode
/usr/share/emacs/site-lisp/elpa/py-isort-2016.1/py-isort hides 
/usr/share/emacs/site-lisp/elpa-src/py-isort-2016.1/py-isort
/usr/share/emacs/site-lisp/elpa/py-isort-2016.1/py-isort-autoloads hides 
/usr/share/emacs/site-lisp/elpa-src/py-isort-2016.1/py-isort-autoloads
/usr/share/emacs/site-lisp/elpa/py-isort-2016.1/py-isort-pkg hides 
/usr/share/emacs/site-lisp/elpa-src/py-isort-2016.1/py-isort-pkg
/home/xiyueden/.config/emacs/elpa/transient-0.5.2/transient hides 
/usr/share/emacs/29.1/lisp/transient

Features:
(shadow emacsbug cl-print shortdoc tex-info tex texmathp texinfo
texinfo-loaddefs help-fns radix-tree magit-patch magit-subtree
magit-gitignore magit-ediff ediff ediff-merg ediff-mult ediff-wind
ediff-diff ediff-help ediff-init ediff-util make-mode
debian-control-mode debian-bug debian-changelog-mode goto-addr misearch
multi-isearch apropos url-http url-gw url-auth url-queue url-cache
shr-color vterm magit-bookmark bookmark tramp tramp-loaddefs trampver
tramp-integration files-x tramp-compat term ehelp vterm-module
git-rebase eglot external-completion array jsonrpc ert ewoc debug
backtrace find-func magit-extras face-remap magit-submodule
magit-obsolete magit-blame magit-stash magit-reflog magit-bisect
magit-push magit-pull magit-fetch magit-clone magit-remote magit-commit
magit-sequence magit-notes magit-worktree magit-tag magit-merge
magit-branch magit-reset magit-files magit-refs magit-status magit
magit-repos magit-apply magit-wip magit-log which-func imenu magit-diff
smerge-mode diff git-commit log-edit add-log magit-core magit-autorevert
magit-margin magit-transient magit-process with-editor shell pcomplete
magit-mode transient edmacro kmacro compat format-spec magit-git
magit-section mailalias dired-aux flow-fill qp matlab matlab-scan
matlab-syntax matlab-compat pulse color mm-archive magit-utils crm dash
mule-util jka-compr sort gnus-cite mail-extr textsec uni-scripts
idna-mapping ucs-normalize uni-confusable textsec-check gnus-async
gnus-bcklg gnus-ml gnus-topic cursor-sensor utf-7 nnfolder gnus-demon
nnml ezgnus gnus-delay gnus-draft gnus-agent gnus-srvr gnus-score
score-mode nnvirtual nntp gnus-cache gnus-msg gnus-art mm-uu mml2015
mm-view mml-smime smime dig gnus-sum shr pixel-fill kinsoku url-file svg
dom nndraft nnmh gnus-group gnus-undo smtpmail gnus-start gnus-dbus dbus
xml gnus-cloud nnimap nnmail mail-source utf7 nnoo gnus-spec gnus-int
gnus-range message sendmail yank-media dired dired-loaddefs rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 rfc2047 rfc2045
ietf-drums mailabbrev gmm-utils mailheader gnus-win gnus nnheader
gnus-util mail-utils range mm-util mail-prsvr windmove flyspell ispell
gnutls network-stream puny nsm epa-file epa derived epg rfc6068
epg-config rcirc parse-time iso8601 time-date term/xterm xterm comp
comp-cstr rx server cap-words superword subword vc-hg vc-git diff-mode
vc-bzr vc-src vc-sccs vc-svn vc-cvs vc-rcs log-view pcvs-util vc
vc-dispatcher bug-reference disp-table whitespace yasnippet cus-edit pp
cus-start wid-edit company-oddmuse company-keywords company-etags etags
fileloop generator xref company-gtags company-dabbrev-code
company-dabbrev company-files company-clang company-capf company-cmake
company-semantic company-template company-bbdb company pcase init
zenburn-theme treesit-auto treesit keychain-environment
exec-path-from-shell icomplete cus-load flymake-proc flymake project
compile text-property-search comint ansi-osc ansi-color ring warnings
icons thingatpt advice cl-extra help-mode use-package use-package-ensure
use-package-delight use-package-diminish use-package-bind-key bind-key
easy-mmode use-package-core display-line-numbers autorevert filenotify
apache-mode-autoloads auctex-autoloads tex-site bison-mode-autoloads
boxquote-autoloads cargo-autoloads cmake-mode-autoloads
company-autoloads csv-mode-autoloads dart-mode-autoloads
exec-path-from-shell-autoloads flutter-autoloads format-all-autoloads
git-modes-autoloads gnuplot-autoloads go-mode-autoloads
graphviz-dot-mode-autoloads inheritenv-autoloads
keychain-environment-autoloads language-id-autoloads magit-autoloads
git-commit-autoloads magit-section-autoloads dash-autoloads
matlab-mode-autoloads meson-mode-autoloads nginx-mode-autoloads
pyvenv-autoloads rust-mode-autoloads scala-mode-autoloads
transient-autoloads treesit-auto-autoloads vterm-autoloads
with-editor-autoloads compat-autoloads xclip-autoloads
yaml-mode-autoloads yasnippet-autoloads zenburn-theme-autoloads info
debian-el-autoloads dpkg-dev-el-autoloads package browse-url url
url-proxy url-privacy url-expand url-methods url-history url-cookie
generate-lisp-file url-domsuf url-util mailcap url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs password-cache json subr-x
map byte-opt gv bytecomp byte-compile url-vars cl-loaddefs cl-lib rmc
iso-transl tooltip cconv eldoc paren electric uniquify ediff-hook
vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode lisp-mode prog-mode register
page tab-bar menu-bar rfn-eshadow isearch easymenu timer select
scroll-bar mouse jit-lock font-lock syntax font-core term/tty-colors
frame minibuffer nadvice seq simple cl-generic indonesian philippine
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite emoji-zwj charscript
charprop case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting cairo move-toolbar gtk x-toolkit xinput2 x multi-tty
make-network-process native-compile emacs)

Memory information:
((conses 16 1373649 182645)
 (symbols 48 37295 55)
 (strings 32 173976 29539)
 (string-bytes 1 6912179)
 (vectors 16 101557)
 (vector-slots 8 2425275 106882)
 (floats 8 704 2874)
 (intervals 56 53218 7615)
 (buffers 984 90))