Improve Windows time.clock() blurb; was missing return type and unit.

Doc/lib/libtime.tex

@@ -134,10 +134,10 @@ on that of the C function of the same name, but in any case, this is
 the function to use for benchmarking\index{benchmarking} Python or
 timing algorithms.
 
-On Windows, this function returns the nearest approximation to
-wall-clock time since the first call to this function, based on the
-Win32 function \cfunction{QueryPerformanceCounter()}.  The resolution
-is typically better than one microsecond.
+On Windows, this function returns wall-clock seconds elapsed since the
+first call to this function, as a floating point number,
+based on the Win32 function \cfunction{QueryPerformanceCounter()}.
+The resolution is typically better than one microsecond.
 \end{funcdesc}
 
 \begin{funcdesc}{ctime}{\optional{secs}}