Clarified documentation relating to usage of serialized_rollback for certain classes

Any initial data loaded in migrations will only be available in ``TestCase``

tests and not in ``TransactionTestCase`` tests, and additionally only on

backends where transactions are supported (the most important exception being

MyISAM).

MyISAM). This is also true for tests which rely on ``TransactionTestCase``

such as :class:`LiveServerTestCase` and

:class:`~django.contrib.staticfiles.testing.StaticLiveServerTestCase`.

Django can reload that data for you on a per-testcase basis by

setting the ``serialized_rollback`` option to ``True`` in the body of the