From OmniFaces Vdldoc' homepage:
Vdldoc is a forked and rewritten version of old JSP TLDdoc Generator as previously available at http://taglibrarydoc.dev.java.net (which is nowhere available right now). Vdldoc has near-complete support for Facelets *.taglib.xml files. The generated documentation has Java 8 javadoc look'n'feel.
It looks similar to documentation generated by JavaDoc. OmniFaces has great documentation to look at.