Added reference to AMK's HOWTO on the topic.

Doc/lib/librestricted.tex

@@ -72,3 +72,8 @@ environments:
 --- Providing restricted access to objects.
 
 \end{description}
+
+\begin{seealso}
+\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.''  Available
+online at \url{http://www.python.org/doc/howto/rexec/}.}
+\end{seealso}

Doc/librestricted.tex

@@ -72,3 +72,8 @@ environments:
 --- Providing restricted access to objects.
 
 \end{description}
+
+\begin{seealso}
+\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.''  Available
+online at \url{http://www.python.org/doc/howto/rexec/}.}
+\end{seealso}