Document search/query languages in the Reference Guide

We probably want to have a section/chapter in the Reference Guide that presents the different languages. We probably just need to have a bit of intro (list the different languages, maybe show how to use them via JCR API), and then describe the different languages in detail. The SQL language and full-text search parsers have pretty good JavaDoc, and we can probably just lift it and reformat it for DocBook. And for XPath, we should be able to just reference the appropriate sections in the JCR 1.0 specification (perhaps linking to the online versions hosted by Day).