Improve documentation of several recent changes

Improve documentation of several recent changes

* src/xfaces.c (merge_face_ref): Fix a typo in the commentary.
(evaluate_face_filter, filter_face_ref): Minor copyedits in
the commentary.
* doc/lispref/display.texi (Face Remapping):
* doc/lispref/text.texi (Special Properties): Document the
':filter' face specs and their effects.  Document
'face-filters-always-match'.

* doc/emacs/files.texi (Visiting): Document the new
possibility to visit large files literally in response to
question asked by Emacs.
* etc/NEWS: Mention the new possibility to visit large files
literally.
* lisp/files.el (files--ask-user-about-large-file): Use
"literally" instead of "raw", for consistency with
find-file-literally.

* doc/lispref/frames.texi (Input Focus): Tell explicitly that
focus-change events are sometimes supported on TTY frames.