bug#50041: Add font-lock-doc-markup-face

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Fri, 13 Aug 2021 23:24:13 +0200
> Cc: larsi@gnus.org, 50041@debbugs.gnu.org
> 
> >  "Font Lock mode face used to highlight documentation."
> 
> All faces have very terse doc strings and I naturally assumed that was the 
> preferred style; it certainly didn't prevent me from understanding it. 
> Anyway, I have amended the doc strings of both font-lock-doc-face and 
> font-lock-doc-markup-face.
> 
> > Also, I think "for mark-up elements" is too terse and assumes too much
> > of prior knowledge; we should make more clear what that means, or
> > provide a couple of examples.
> 
> Maybe, but let's not exaggerate; this is standard terminology. Nevertheless, 
> the text has been extended.
> 
> > The NEWS entry is now clear enough, but I think the manual
> > and the doc string still need some work.
> 
> Very well, they have now all been expanded further.

Thanks, this LGTM.