bug#32680: 27.0.50; Improve docstring and customization for 'message-signature'

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Mon, 10 Sep 2018 16:25:21 +0200
> Cc: 32680@debbugs.gnu.org
> 
> Philipp Stephani <p.stephani2@gmail.com> writes:
> 
> > The manual states
> >
> >    If you change ‘message-signature’ to ‘nil’, Emacs will not insert
> > your mail signature automatically.  You can insert your mail signature
> > by typing ‘C-c C-w’ (‘message-insert-signature’) in the mail buffer.
> > Emacs will look for your signature in the signature file.
> >
> > However, the docstring for message-signature doesn't state that nil is a
> > possible value, and the customization choices also don't suggest nil.
> 
> We could even put this in emacs-26.

Fine with me, but if it will go to emacs-26, then the :version tag
should say so.

Thanks.