Fix docstring of format-time-string to include %F.

See discussion of Bug#15816.

* editfns.c (Fformat_time_string): Mention %F in the doc.

src/ChangeLog

+2013-12-11  Rüdiger Sonderfeld  <ruediger@c-plusplus.de>
+
+	* editfns.c (Fformat_time_string): Mention %F in the doc.
+
 2013-12-11  Martin Rudalics  <rudalics@gmx.at>
 
 	* window.c (resize_frame_windows): Don't return immediately when

src/editfns.c

@@ -1723,6 +1723,7 @@ by text that describes the specified date and time in TIME:
 %c is the locale's date and time format.
 %x is the locale's "preferred" date format.
 %D is like "%m/%d/%y".
+%F is the ISO 8601 date format (like "%Y-%m-%d").
 
 %R is like "%H:%M", %T is like "%H:%M:%S", %r is like "%I:%M:%S %p".
 %X is the locale's "preferred" time format.
@@ -1741,7 +1742,7 @@ The modifiers are `E' and `O'.  For certain characters X,
 %EX is a locale's alternative version of %X;
 %OX is like %X, but uses the locale's number symbols.
 
-For example, to produce full ISO 8601 format, use "%Y-%m-%dT%T%z".
+For example, to produce full ISO 8601 format, use "%FT%T%z".
 
 usage: (format-time-string FORMAT-STRING &optional TIME UNIVERSAL)  */)
   (Lisp_Object format_string, Lisp_Object timeval, Lisp_Object universal)