Add doc for timegm().

Doc/lib/libcalendar.tex

@@ -46,3 +46,11 @@ week will use.
 \begin{funcdesc}{prcal}{year}
 Prints the calendar for the year \var{year}.
 \end{funcdesc}
+
+\begin{funcdesc}{timegm}{tuple}
+An unrelated but handy function that takes a time tuple such are
+returned by the \function{gmtime()} function in the \module{time}
+module, and returns the corresponding Unix timestamp value, assuming
+an epoch of 1970, and the POSIX encoding.  In fact,
+\function{gmtime()} and \function{timegm()} are each others inverse.
+\end{funcdesc}