Improve documentation of faces in the user manual.

 doc/emacs/frames.texi (Scroll Bars): Improve indexing of faces.
 doc/emacs/killing.texi (Secondary Selection): Improve indexing of faces.
 doc/emacs/search.texi (Basic Isearch, Query Replace): Improve indexing of
 faces.
 doc/emacs/display.texi (Standard Faces, Text Display)
 (Useless Whitespace): Improve indexing of faces.