bug#11934: 24.1; provide variable for pp.el to control max display width

[Drew Adams]

Enhancement request.
 
The functions in pp.el that produce pretty-printed output are used for
things like `describe-variable' to produce output that is read in *Help*
buffers.  The content of *Help* is normally limited in width.
 
The request is to provide a variable that controls the width of the
displayed expression, or at least tries to as much as possible.
 
For example, I have a variable whose value is this:
 
(bbdb-complete-name comint-completion-at-point
comint-dynamic-complete-filename comint-replace-by-expanded-filename
ess-complete-object-name gud-gdb-complete-command Info-goto-node
Info-index Info-menu lisp-complete-symbol lisp-completion-at-point
minibuffer-default-add-completions read-char-by-name read-color
read-from-minibuffer read-string recentf-make-menu-items)
 
In Message mode, where I am composing this, that is automatically
filled.  But with the pp functions, that value is written with no
newline chars, as a single line that is 369 chars wide.  That sticks
out like a sore thumb in a *Help* buffer that is otherwise designed
to be limited in width.
 
It would be good to be able to bind a max-width variable that lets the
pp functions know that it is better, if possible, to insert newline
chars to try to keep the width below that var's value.
 
I'm not sure what the implementation would look like.  Perhaps it would
involve calling `fill-paragraph' at various points
(`lisp-fill-paragraph' would add nothing here, AFAICT).  Dunno.
 
But it seems like we could somehow do better in a case like this than
just print everything on a single line, which might be hundreds of chars
wide.
 
 
 
In GNU Emacs 24.1.1 (i386-mingw-nt5.1.2600)
 of 2012-06-10 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include'