Fixed #20316 - Clarified transaction behavior of TestCase.

* A ``TestCase``, on the other hand, does not truncate tables after a test.

  Instead, it encloses the test code in a database transaction that is rolled

  back at the end of the test.  It also prevents the code under test from

  issuing any commit or rollback operations on the database, to ensure that the

  rollback at the end of the test restores the database to its initial state.

  back at the end of the test. Both explicit commits like

  ``transaction.commit()`` and implicit ones that may be caused by

  ``Model.save()`` are replaced with a ``nop`` operation. This guarantees that

  the rollback at the end of the test restores the database to its initial

  state.

  When running on a database that does not support rollback (e.g. MySQL with the

  MyISAM storage engine), ``TestCase`` falls back to initializing the database

  by truncating tables and reloading initial data.

.. warning::

    While ``commit`` and ``rollback`` operations still *appear* to work when

    used in ``TestCase``, no actual commit or rollback will be performed by the

    database. This can cause your tests to pass or fail unexpectedly. Always

    use ``TransactionalTestCase`` when testing transactional behavior.

.. note::

    .. versionchanged:: 1.5