Document how the hibernate session is implemented

There have been many questions on the user's list about how the hibernate session is implemented in tapestry-hibernate and also how to use tapestry-hibernate outside of a tapestry managed request / response.

I think that the documentation should mention the following:
1. The hibernate session service provided by tapestry-hibernate is a singleton
2. The singleton is a proxy that points to a per-thread, lazy loaded hibernate session instance
3. The per-thread session instance is cleaned up by Registry.cleanupThread() /
PerThreadManager.cleanup()
4. Tapestry automatically cleans up the thread local inside the normal request / response flow
5. Outside of a tapestry managed request / response, you must explicitly cleanup the thread
6. Transaction management outside of a request/response flow
a. @CommitAfter and @Match("*DAO") with HibernateTransactionAdvisor
b. HibernateSessionManager.commit() and abort()
c. session.getTransaction().commit()