- Added after_soft_rollback() Session event. This

   - Added after_soft_rollback() Session event.  This
     event fires unconditionally whenever rollback()
     is called, regardless of if an actual DBAPI
     level rollback occurred.  This event
    is specifically designed to allow operations
     with the Session to proceed after a rollback
     when the Session.is_active is True.
     [ticket:2241]
- SessionTransaction is mentioned in public docs, many
more docstrings for events etc. otherwise