Ensure autobegin occurs for attribute changes; Document autobegin

Ensure autobegin occurs for attribute changes; Document autobegin

The Session autobegin feature was not anticipated as having
any behavioral changes other than the event hook being called
at a different time, however as autobegin impacts the behavior
of the commit() and rollback() methods in that they can now
be no-ops in non-autocommit mode, document the behavior fully.

Fixed issue where the new :ref:`session_autobegin` behavior failed to
"autobegin" in the case where an existing persistent object has an
attribute change, which would then impact the behavior of
:meth:`_orm.Session.rollback` in that no snapshot was created to be rolled
back. The "attribute modify" mechanics have been updated to ensure
"autobegin", which does not perform any database work, does occur when
persistent attributes change in the same manner as when
:meth:`_orm.Session.add` is called. This is a regression as in 1.3, the
rollback() method always had a transaction to roll back and would expire
every time.

Fixes: #6360
Fixes: #6359
Change-Id: I69f231a206f49e3231275d23bbe2cafd4e2bf3ba