- A clear error message is emitted if an event handler

- A clear error message is emitted if an event handler
attempts to emit SQL on a Session within the after_commit()
handler, where there is not a viable transaction in progress.
[ticket:2662]
- rework how SessionTransaction maintains state, using symbols
instead.
- add lots of notes and cross-linking for session events.
- add a link to :func:`.select()` within :meth:`.FromClause.select`.