bug#67638: Description of the variable message-mail-user-agent in Emacs 29.1

[Eli Zaretskii]

> Cc: 67638@debbugs.gnu.org
> Date: Thu, 07 Dec 2023 19:40:31 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > > rmail-start-mail (and thus compose-mail) is also called by commands 
> > > rmail-mail, rmail-continue, rmail-forward, and a couple of others.
> > 
> > Thanks. Can you please explain where the variable 
> > message-mail-user-agent intervenes in the process above? Because only 
> > mail-user-agent appears in your description.

Oh, and message-setup binds mail-user-agent to a value derived from
message-mail-user-agent.  Maybe this is the missing piece you were
looking for.