Updates to Display chapter of Lisp manual.

Updates to Display chapter of Lisp manual.

* doc/lispref/display.texi (The Echo Area): Add xref to Output Streams.
(Displaying Messages): Improve doc of message.
(Echo Area Customization, Invisible Text): Copyedits.
(Invisible Text): Mention that spec comparison is done with eq.
(Width): Improve doc of char-width.
(Faces): Recommend using symbol instead of string for face name.
Minor clarifications.
(Defining Faces): Copyedits.  Update face example.
(Attribute Functions): Mark set-face-foreground etc as commands.
(Face Remapping): Mention text-scale-adjust.  Clarify
face-remapping-alist and related docs.
(Face Functions): Don't document make-face or copy-face.

* lisp/faces.el (make-face, make-empty-face, copy-face):
* lisp/face-remap.el (face-remap-add-relative, face-remap-set-base):
Doc fixes.

* src/xfaces.c (Vface_remapping_alist): Doc fix.