There is a glorious example of systems documentation of the LiveJournal system here. It includes how to install, administer, and program the system. Further, the documentation was generated with DocBook and XSL stylesheets, which means that it is relatively easy to produce documentation that looks exactly like this. Another benefit of using something like DocBook is that pdf versions of the documentation can be generated directly. Maintenance of the live document is easy with DocBook, can be delegated to various people, and glossaries and indexes can be generated automatically.