emacs-28 f1c1fd7184 1/2: Improve documentation of mail-user-agent.

[Eli Zaretskii]

branch: emacs-28
commit f1c1fd718478823b760020ed136345d94f379801
Author: Damien Cassou <damien@cassou.me>
Commit: Eli Zaretskii <eliz@gnu.org>

    Improve documentation of mail-user-agent.
    
    * doc/emacs/sending.texi (Mail Methods):
    * lisp/simple.el (mail-user-agent): Mention additional options
    of non-bundled MUA.  (Bug#5569)
---
 doc/emacs/sending.texi | 4 +++-
 lisp/simple.el         | 7 ++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/doc/emacs/sending.texi b/doc/emacs/sending.texi
index ea87e0a77b..acdae8a263 100644
--- a/doc/emacs/sending.texi
+++ b/doc/emacs/sending.texi
@@ -667,7 +667,9 @@ the bug reporter (@pxref{Bugs}).  To specify a mail user 
agent,
 customize the variable @code{mail-user-agent}.  Currently, legitimate
 values include @code{message-user-agent} (Message mode)
 @code{sendmail-user-agent} (Mail mode), @code{gnus-user-agent}, and
-@code{mh-e-user-agent}.
+@code{mh-e-user-agent}.  Additional options may be available; check
+in the manual of your mail user agent package for details.  You may
+also define another mail user agent using `define-mail-user-agent'.
 
   If you select a different mail-composition method, the information
 in this chapter about the mail buffer and Message mode does not apply;
diff --git a/lisp/simple.el b/lisp/simple.el
index b9cb957064..d235eb9745 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -8735,9 +8735,10 @@ Valid values include:
                            paraphernalia if Gnus is running, particularly
                            the Gcc: header for archiving.
 
-Additional valid symbols may be available; check with the author of
-your package for details.  The function should return non-nil if it
-succeeds.
+Additional valid symbols may be available; check in the manual of
+your mail user agent package for details.  You may also define
+your own symbol to be used as value for this variable using
+`define-mail-user-agent'.
 
 See also `read-mail-command' concerning reading mail."
   :type '(radio (function-item :tag "Message package"