On this page:
Documentation for XSLT-elements and predefined XPath-functions
The plugin comes with bundled documentation for XSLT elements and functions that is accessible via the Quick Documentation Lookup (Ctrl-Q) action. This documentation is extracted from the respective sections in the official W3C XSLT and XPath documents.
It's also possible to annotate elements in an XSLT stylesheet with some documentation fragments. This can be looked up by the Quick Documentation Lookup function for variables, parameters and templates