perldoc XML::LibXML prints something that definitely doesn't fit a single page and
uh, the Parser is documented in XML::LibXML::Parser.
And XPath? Where's that even just mentioned in the docs?
Are you asking where it's documented that XML::LibXML::Node::findnodes takes an XPath, or are you asking where XPaths are documented in XML::LibXML.
If the former, XML::LibXML::Node.
If the latter, your argument is ridiculous. XML::LibXML is no more the place to document XPaths than XML::Simple is the place to document references.