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

[Damien Cassou <damien@cassou.me>]

Hi Eli,

Eli Zaretskii <eliz@gnu.org> writes:
>> +@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'.
>
> Thanks, but what do you mean by "the author of your mail user agent
> package"?


I mean that the authors of notmuch and mu4e both define a symbol to be
used as value for the `mail-user-agent` variable: `mu4e-user-agent` and
`notmuch-user-agent`.  These symbols should be documented somewhere in
notmuch and mu4e manuals so their users know they should pass them to
`mail-user-agent`.  There might be other mail user agent packages out
there that I don't know about.


> This is an Emacs user manual, and I very much doubt that
> Emacs users could figure out where and how to check that.
>
> Can we provide more practical advice here how to check for those
> additional options?


I could certainly mention the symbols for notmuch and mu4e if you think
that is ok to mention external packages inside the Emacs manual but I'm
not sure what else I could do.

-- 
Damien Cassou

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