bug#29405: 27.0.50; message-faces: Doc fixes and syntax touch-ups

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Date: Wed, 22 Nov 2017 20:48:28 +0000
> 
> I attach three patches pertaining to the faces defined in
> lisp/gnus/message.el in descending order of importance:
> 
> 1. Fix docstring mistakes and inconsistencies.
> 
> 2. Use define-obsolete-face-alias instead of duplicating it.
> 
> 3. Use the newer (DISPLAY . PLIST) face spec syntax,
>    as per `(elisp) Defining Faces'.

Lars, Yamaoka-san -- any comments on these?

I've installed the first one on the emacs-26 branch, as it's clearly a
bugfix in doc strings.  But I would like to hear your opinions on the
rest.

Thanks.