Contributing to Geronimo's documentation is as easy as easy can get!
We use a Confluence wiki to develop the project's documentation, then we massage the wiki content with a Confluence plugin to generate a custom HTML version. That is what you see when you access http://cwiki.apache.org/geronimo
So, if you are getting this far you are probably monitoring the User and the Development mailing lists already. The issues discussed on these list are mostly driving what topics need to be covered by the documentation. So, the mailing lists are a great starting point for discussing what areas you would like to see covered or can directly contribute to. User experiences are a great asset for the documentation.
Geronimo documentation is somewhat unique when compared to other projects. This is in the sense that anybody can contribute directly, no need to be a committer before hand or creating a JIRA in order to add content. Contributing is pretty much straight forward however, it is very important that you discuss your intentions on the mailing lists so we can maintain some consistency across the rest of the documentation.
For that purpose there is a guide Tips for writing and formatting documentation that should help you answer some basic questions. The Geronimo cwiki documentation architecture page gives you an overview of how we are using the wiki to build up Geronimo's documentation.
Geronimo's documentation is a community effort, it is based on contributions from the users to the users. Your contributions make the difference.
So, c'on in, jump into the mailing lists and bring your ideas forward!