The source for the HBase Reference Guide is in the HBase source, in the src/main/docbkx/ directory. It is written in Docbook XML. Docbook can be intimidating, but you can typically follow the formatting of the surrounding file to get an idea of the mark-up. You can edit Docbook XML files using a plain-text editor, an XML-aware IDE, or a specialized XML editor.

Docbook's syntax can be picky. Before submitting a patch, be sure to build the output locally using the mvn site command. If you do not get any build errors, that means that the XML is well-formed, which means that each opening tag is balanced by a closing tag. Well-formedness is not exactly the same as validity. Check the output in target/docbkx/ for any surprises before submitting a patch.