Document calendar-day-header-array

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

* etc/NEWS: Markup.

doc/emacs/ChangeLog

+2014-01-07  Glenn Morris  <rgm@gnu.org>
+
+	* cal-xtra.texi (Calendar Customizing):
+	Mention calendar-day-header-array.
+
 2013-12-28  Glenn Morris  <rgm@gnu.org>
 
 	* trouble.texi (Understanding Bug Reporting): Brevity.

doc/emacs/cal-xtra.texi

@@ -43,9 +43,12 @@ customize the variables @code{calendar-intermonth-header} and
 @code{calendar-intermonth-text} as described in their documentation.
 
 @vindex calendar-month-header
+@vindex calendar-day-header-array
   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.
+month and year.  The variable @code{calendar-day-header-array}
+controls the text that appears above each day's column in every month.
+By default, it shows the first two letters of each day's name.
 
 @vindex calendar-holiday-marker
 @vindex diary-entry-marker

etc/NEWS

@@ -353,12 +353,14 @@ code run via JIT Lock.
 *** New faces `calendar-weekday-header', `calendar-weekend-header',
 and `calendar-month-header'.
 
++++
 *** New option `calendar-day-header-array'.
 
 +++
 *** New variable `diary-from-outlook-function', used by the command
 `diary-from-outlook'.
 
+---
 *** The variable `calendar-font-lock-keywords' is obsolete.
 
 ** Calc