Improve documentation of Auto-Revert mode

* doc/emacs/files.texi (Reverting): Mention that Aut-Revert
applies to Dired buffers as well.  Suggested by Michael Albinus
<michael.albinus@gmx.de> in emacs-manual-bugs@gnu.org.

doc/emacs/files.texi

@@ -988,6 +988,9 @@ Auto-Revert Tail mode works also for remote files.
   When a buffer is auto-reverted, a message is generated.  This can be
 suppressed by setting @code{auto-revert-verbose} to @code{nil}.
 
+  In Dired buffers (@pxref{Dired}), Auto-Revert mode refreshes the
+buffer when a file is created or deleted in the buffer's directory.
+
   @xref{VC Undo}, for commands to revert to earlier versions of files
 under version control.  @xref{VC Mode Line}, for Auto Revert
 peculiarities when visiting files under version control.