The odds are pretty good that you don't want to use a generic tool for documenting your schema. Schema documentation tools (I'm most familiar with the one in XML Spy) are designed to make schemas readable. From your description, it sounds like that's not really what you want; what you want is to use the information in your schema to as the basis of software documentation.
When I've needed to do this, the approach I took was to design HTML that looked the way I needed the documentation to look, and then figure out what information in the schema I'd need to grab to populate it. Using that as a template, it's pretty straightforward to pull in relevant information from the schema using XSLT.
