Edits to Display chapter of Emacs manual.

Edits to Display chapter of Emacs manual.

* doc/emacs/display.texi (Faces): Simplify discussion.  Move documentation
of list-faces-display here, from Standard Faces node.  Note
special role of `default' background.
(Standard Faces): Note special role of `default' background.  Note
that region face may be taken fom GTK.  Add xref to Text Display.
(Text Scale): Rename from "Temporary Face Changes".  Callers
changed.  Don't bother documenting variable-pitch-mode.
(Font Lock): Copyedits.  Remove font-lock-maximum-size.
(Useless Whitespace): Simplify description of
delete-trailing-whitespace.  Note active region case.
(Text Display): Fix description of escape-glyph face assignment.
Remove unibye mode discussion.  Update some parts for Unicode.
Move glyphless chars documentation to Lisp manual.

* doc/emacs/frames.texi (Tooltips): Document x-gtk-use-system-tooltips.

* doc/lispref/display.texi (Glyphless Chars): New node.