PhpStorm 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 Style Sheet with documentation fragments. This can be looked up by the Quick Documentation Lookup function for variables, parameters, and templates
Before you start
Install and enable the XPathView + XSLT plugin on the Settings/Preferences | Plugins page, tab Marketplace, as described in Installing plugins from JetBrains repository.