Improve 'read-face-name'

Improve 'read-face-name'

* doc/lispref/display.texi (Face Functions): Use '@lisp' for Elisp
example.  Document 'read-face-name' and 'read-face-name-sample-text'.
* etc/NEWS: Announce 'read-face-name-sample-text' as a user option.
* lisp/faces.el (read-face-name-sample-text): Make it a user option.
(completion-face-name-affixation, read-face-name-sort-aliases-last)
(completion-face-name-table): New functions.
(read-face-name): Simplify.