backport weekheader() doc fix to 2.3

Doc/lib/libcalendar.tex

@@ -58,6 +58,11 @@ Returns the day of the week (\code{0} is Monday) for \var{year}
 (\code{1}--\code{31}).
 \end{funcdesc}
 
+\begin{funcdesc}{weekheader}{n}
+Return a header containing abbreviated weekday names. \var{n} specifies
+the width in characters for one weekday.
+\end{funcdesc}
+
 \begin{funcdesc}{monthrange}{year, month}
 Returns weekday of first day of the month and number of days in month, 
 for the specified \var{year} and \var{month}.