Document calendar-month-header

* doc/emacs/cal-xtra.texi (Calendar Customizing): Mention calendar-month-header.

* etc/NEWS: Related edit.

doc/emacs/ChangeLog

 2012-10-08  Glenn Morris  <rgm@gnu.org>
 
+	* cal-xtra.texi (Calendar Customizing): Mention calendar-month-header.
+
 	* calendar.texi (Writing Calendar Files): Mention cal-html-holidays.
 
 2012-10-06  Glenn Morris  <rgm@gnu.org>

doc/emacs/cal-xtra.texi

@@ -41,6 +41,11 @@ To display text @emph{between} the months, for example week numbers,
 customize the variables @code{calendar-intermonth-header} and
 @code{calendar-intermonth-text} as described in their documentation.
 
+@vindex calendar-month-header
+  The variable @code{calendar-month-header} controls the text that
+appears above each month in the calendar.  By default, it shows the
+month and year.
+
 @vindex calendar-holiday-marker
 @vindex diary-entry-marker
 @vindex calendar-today-marker

etc/NEWS

@@ -280,6 +280,7 @@ Use `Buffer-menu-name-width' and `Buffer-menu-size-width' instead.
 
 ** Calendar
 
++++
 *** You can customize the header text that appears above each calendar month.
 See the variable `calendar-month-header'.