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

[Drew Adams]

This is a useful, general function, not just a helper.
Please consider giving it a better name and doc string.

Wrt the doc, the SPEC arg is apparently NOT a face spec, i.e., a spec
such as is returned by (get FACE 'face-defface-spec).  Instead, it seems
to be an attributes list such as what is returned by `face-spec-choose'
or `face-attr-construct'.

In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'