facemenu-set-face doc string

[Drew Adams]

The current explanation is not very clear, IMO (just what are "the faces to
use"? apply a face to faces?).

I propose a minor change: use active voice and refer to
`facemenu-add-new-face' and `facemenu-add-face', whose doc strings are
clear.

OLD:

 Apply FACE to the region or next character typed.

 If the region is active (normally true except in Transient
 Mark mode) and nonempty, and there is no prefix argument,
 this command applies FACE to the region.  Otherwise, it applies FACE
 to the faces to use for the next character
 inserted.  (Moving point or switching buffers before typing
 a character to insert cancels the specification.)

 If FACE is `default', to \"apply\" it means clearing
 the list of faces to be used.  For any other value of FACE,
 to \"apply\" it means putting FACE at the front of the list
 of faces to be used, and removing any faces further
 along in the list that would be completely overridden by
 preceding faces (including FACE).

 This command can also add FACE to the menu of faces,
 if `facemenu-listed-faces' says to do that.

PROPOSED:

 Apply FACE to the region or the next character typed.
 If there is no prefix argument and the region is active and nonempty,
 then apply FACE to the region.  Otherwise, apply FACE to the next
 character inserted.

 Call `facemenu-add-new-face', then `facemenu-add-face'.