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

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, <yamaoka@jpl.org>, 
> <29405@debbugs.gnu.org>
> Date: Wed, 13 Dec 2017 01:19:13 +0000
> 
> Sorry, I managed to omit the majority of face names from the patch which
> updates the face spec syntax, thanks to a combination of rushing and
> text collapsing.  Kindly find attached a revised version.

Thanks, pushed.