Doc updates related to changes in revert-buffer function variables

Doc updates related to changes in revert-buffer function variables

* lisp/files.el (make-backup-file-name-function)
(make-backup-file-name, make-backup-file-name--default-function)
(make-backup-file-name-1, find-backup-file-name)
(revert-buffer-function, revert-buffer-insert-file-contents-function)
(buffer-stale--default-function, buffer-stale-function)
(before-revert-hook, after-revert-hook, revert-buffer-in-progress-p)
(revert-buffer, revert-buffer--default)
(revert-buffer-insert-file-contents--default-function):
Doc fixes related to defaults no longer being nil.
(make-backup-file-name-function): Bump :version.
Restore nil as a valid but deprecated custom type.

* etc/NEWS: Related edit.