Refer reader to platform library docs for specific semantics.

Resolves SF bugs 697989, 697988, 697986.

Doc/lib/libtime.tex

@@ -5,9 +5,12 @@
 \modulesynopsis{Time access and conversions.}
 
 
-This module provides various time-related functions.
-It is always available, but not all functions are available
-on all platforms.
+This module provides various time-related functions.  It is always
+available, but not all functions are available on all platforms.  Most
+of the functions defined in this module call platform C library
+functions with the same name.  It may sometimes be helpful to consult
+the platform documentation, because the semantics of these functions
+varies among platforms.
 
 An explanation of some terminology and conventions is in order.