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

[Eli Zaretskii]

> From: Damien Cassou <damien@cassou.me>
> Date: Sun, 22 May 2022 08:16:25 +0200
> 
> 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'.

Thanks, but what do you mean by "the author of your mail user agent
package"?  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?