add seealso referring to site module doc where people can learn how to

extend sys.path using .pth files.

Doc/lib/libsys.tex

@@ -482,3 +482,9 @@ else:
   Python.
   Availability: Windows.
 \end{datadesc}
+
+
+\begin{seealso}
+  \seemodule{site}
+    {This describes how to use .pth files to extend \code{sys.path}.}
+\end{seealso}