bug#55569: Improve documentation of mail-user-agent

[Damien Cassou <damien@cassou.me>]

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Tags: patch

* doc/emacs/sending.texi (Mail Methods): Add sentences.
* lisp/simple.el (mail-user-agent): Add sentences to the documentation
string.

-- 
Damien Cassou

"Success is the ability to go from one failure to another without
losing enthusiasm." --Winston Churchill

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From be4bcf3ade2b64b211074ba92688922f4f43978e Mon Sep 17 00:00:00 2001
From: Damien Cassou <damien@cassou.me>
Date: Sun, 22 May 2022 08:09:38 +0200
Subject: [PATCH] Improve documentation of mail-user-agent.

* doc/emacs/sending.texi (Mail Methods): Add sentences.
* lisp/simple.el (mail-user-agent): Add sentences to the documentation
string.
---
 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..221d2aa3dd 100644
--- a/doc/emacs/sending.texi
+++ b/doc/emacs/sending.texi
@@ -667,7 +667,9 @@ Mail Methods
 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
+with the author 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 6906675f68..f901da9bc6 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -9210,9 +9210,10 @@ mail-user-agent
                            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 with the author
+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"
-- 
2.36.0