Fix bug #7713 with doc string of display-time-mode.

 time.el (display-time-mode): Mention display-time-interval in
 the doc string.

lisp/ChangeLog

 2010-12-25  Eli Zaretskii  <eliz@gnu.org>
 
+	* time.el (display-time-mode): Mention display-time-interval in
+	the doc string.  (Bug#7713)
+
 	* simple.el (select-active-regions): Doc fix.  (Bug#7702)
 
 2010-12-24  Kenichi Handa  <handa@m17n.org>

lisp/time.el

@@ -462,7 +462,9 @@ update which can wait for the next redisplay."
   "Toggle display of time, load level, and mail flag in mode lines.
 With a numeric arg, enable this display if arg is positive.
 
-When this display is enabled, it updates automatically every minute.
+When this display is enabled, it updates automatically every minute
+\(you can control the number of seconds between updates by
+customizing `display-time-interval').
 If `display-time-day-and-date' is non-nil, the current day and date
 are displayed as well.
 This runs the normal hook `display-time-hook' after each update."