Add a comment on time.time() returning non-decreasing values except when

the clock is set back.

This closes SF bug #447945.

Doc/lib/libtime.tex

@@ -287,7 +287,9 @@ Availability: Most modern \UNIX{} systems.
 Return the time as a floating point number expressed in seconds since
 the epoch, in UTC.  Note that even though the time is always returned
 as a floating point number, not all systems provide time with a better
-precision than 1 second.
+precision than 1 second.  While this function normally returns
+non-decreasing values, it can return a lower value than a previous
+call if the system clock has been set back between the two calls.
 \end{funcdesc}
 
 \begin{datadesc}{timezone}