SF 639138, Ref. calendar module in time docs by John J Lee

Will backport.

Doc/lib/libtime.tex

@@ -322,4 +322,7 @@ timezone is defined, the second string should not be used.
   \seemodule{locale}{Internationalization services.  The locale
                      settings can affect the return values for some of 
                      the functions in the \module{time} module.}
+  \seemodule{calendar}{General calendar-related functions.  
+                       \function{timegm()} is the inverse of
+                       \function{gmtime()} from this module.}
 \end{seealso}