New variable 'revert-buffer-restore-functions' (bug#69511)

New variable 'revert-buffer-restore-functions' (bug#69511)

* doc/lispref/backups.texi (Reverting):
Add documentation for 'revert-buffer-restore-functions'.

* lisp/files.el (revert-buffer-restore-functions): New variable.
(revert-buffer-restore-read-only): New function.
(revert-buffer): Use 'revert-buffer-restore-functions' with
the default value 'revert-buffer-restore-read-only' (bug#69511).

* lisp/buff-menu.el (Buffer-menu-mode): Add hook
'revert-buffer-restore-functions' to restore
outline-minor-mode highlighting.

(cherry picked from commit a525cfb3af0c49c5c64e8af548ab23d086348fed)