bug#27238: 24.5; Rename `face-spec-set-2' and give it a better doc string

[Eli Zaretskii]

> Date: Mon, 5 Jun 2017 12:36:54 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 27238@debbugs.gnu.org
> 
> > The argument received by face-spec-set-2 is just the PLIST
> > which matches the frame's display, i.e. a simple list.
> 
> Yes.  It is not a face spec.  It is a face-attributes plist.

OK, I renamed SPEC to FACE-ATTRS where appropriate, and improved the
related doc strings.

Thanks.