more autocommit messaging

more autocommit messaging

Further clarified connection-level logging to indicate the BEGIN, ROLLBACK
and COMMIT log messages do not actually indicate a real transaction when
the AUTOCOMMIT isolation level is in use; messaging has been extended to
include the BEGIN message itself, and the messaging has also been fixed to
accommodate when the :class:`.Engine` level
:paramref:`.create_engine.isolation_level` parameter was used directly.

Fixes: #7853
Change-Id: Iafc78070737ad117f84262e4bde84b81a81e4ea1