Add references to the documentation for the Python documentation markup.

Suggested by the comments in SF bug #430627.

Doc/templates/howto.tex

+% Complete documentation on the extended LaTeX markup used for Python
+% documentation is available in ``Documenting Python'', which is part
+% of the standard documentation for Python.  It may be found online
+% at:
+%
+%     http://www.python.org/doc/current/doc/doc.html
+
 \documentclass{howto}
 
 %  This is a template for short or medium-size Python-related documents, 

Doc/templates/manual.tex

+% Complete documentation on the extended LaTeX markup used for Python
+% documentation is available in ``Documenting Python'', which is part
+% of the standard documentation for Python.  It may be found online
+% at:
+%
+%     http://www.python.org/doc/current/doc/doc.html
+
 \documentclass{manual}
 
 \title{Big Python Manual}