Point to the PyXML package as an extended version of the "xml" package

that can provide this fuctionality.

Note that the docs for the xml.dom and xml.sax packages are the
definitiona of the Python bindings for the DOM and SAX interfaces.

Doc/lib/markup.tex

@@ -7,4 +7,23 @@ Standard Generalized Markup Language (SGML) and the Hypertext Markup
 Language (HTML), and several interfaces for working with the
 Extensible Markup Language (XML).
 
+It is important to note that modules in the \module{xml} package
+require that there be at least one SAX-compliant XML parser available.
+Starting with Python 2.3, the Expat parser is included with Python, so
+the \refmodule{xml.parsers.expat} module will always be available.
+You may still want to be aware of the \ulink{PyXML add-on
+package}{http://pyxml.sourceforge.net/}; that package provides an
+extended set of XML libraries for Python.
+
+The documentation for the \module{xml.dom} and \module{xml.sax}
+packages are the definition of the Python bindings for the DOM and SAX
+interfaces.
+
 \localmoduletable
+
+\begin{seealso}
+  \seetitle[http://pyxml.sourceforge.net/]
+           {Python/XML Libraries}
+           {Home page for the PyXML package, containing an extension
+            of \module{xml} package bundled with Python.}
+\end{seealso}