Document revert-buffer-in-progress-p

* doc/lispref/backups.texi (Reverting): Mention revert-buffer-in-progress-p.
* lisp/files.el (revert-buffer): Doc fix (mention revert-buffer-in-progress-p).
* etc/NEWS: Related markup.

doc/lispref/ChangeLog

@@ -1,5 +1,7 @@
 2012-02-04  Glenn Morris  <rgm@gnu.org>
 
+	* backups.texi (Reverting): Mention revert-buffer-in-progress-p.
+
 	* debugging.texi (Error Debugging): Mention debug-on-event.
 	* commands.texi (Misc Events): Mention sigusr1,2 and debugging.
 

doc/lispref/backups.texi

@@ -696,6 +696,9 @@ operation, reverting preserves all the markers.  If they are not
 identical, reverting does change the buffer; in that case, it preserves
 the markers in the unchanged text (if any) at the beginning and end of
 the buffer.  Preserving any additional markers would be problematical.
+
+This command binds @code{revert-buffer-in-progress-p} to a
+non-@code{nil} value while it operates.
 @end deffn
 
 You can customize how @code{revert-buffer} does its work by setting

etc/NEWS

@@ -1388,6 +1388,7 @@ time you call `progress-reporter-update' on that progress reporter,
 with a nil or omitted VALUE argument, the reporter message is
 displayed with a "spinning bar".
 
++++
 ** New variable `revert-buffer-in-progress-p' is true while a buffer is
 being reverted, even if the buffer has a local `revert-buffer-function'.
 

lisp/ChangeLog

@@ -1,5 +1,8 @@
 2012-02-04  Glenn Morris  <rgm@gnu.org>
 
+	* files.el (revert-buffer):
+	Doc fix (mention revert-buffer-in-progress-p).
+
 	* emacs-lisp/ert-x.el (ert-simulate-command):
 	Check deferred-action-list (which is obsolete) is bound.
 

lisp/files.el

@@ -5131,6 +5131,8 @@ revert buffers without querying for confirmation.)
 Optional third argument PRESERVE-MODES non-nil means don't alter
 the files modes.  Normally we reinitialize them using `normal-mode'.
 
+This function binds `revert-buffer-in-progress-p' non-nil while it operates.
+
 If the value of `revert-buffer-function' is non-nil, it is called to
 do all the work for this command.  Otherwise, the hooks
 `before-revert-hook' and `after-revert-hook' are run at the beginning