All of Virtualmin’s documentation is written and maintained using DokuWiki. A wiki allows users (just like you!) to help make the documentation better. Consult the DokuWiki syntax page for information on writing wiki pages.
Of course, there are other ways to help make the documentation better. If you can’t find what you’re looking for, file a bug in our bug tracker, and one of the intrepid Virtualmin developers will add the missing pieces, as soon as possible.