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.
> 
> FWIW, I don't think it's general enough, because it is tightly couples
> with face-x-resources.

That's only the _implementation_.

(That reasoning would apply also to `face-spec-recalc', `face-spec-set',
`face-set-after-frame-default', `custom-face-set', `custom-face-save',
`custom-face-mark-to-save', `custom-face-reset-saved',
`custom-face-mark-to-reset-standard', `custom-declare-face',
`custom-theme-set-faces', `custom-reset-faces', `woman-default-faces',
...)

Unless you essentially repeat its body, it is the only way
to do what it does.  Here's one use case:

(face-spec-set-2 TARGET-FACE
                 FRAME
                 (face-spec-choose
                   (get SOURCE-FACE 'face-defface-spec)
                   FRAME))