Clarify usage of serialized_rollback for certain test classes

[This ticket](https://code.djangoproject.com/ticket/23640) highlighted that it is not overly clear that the serialized_rollback attribute should be used for tests inheriting from StaticLiveServerTestCase and LiveServerTestCase. The documentation only referenced TransactionTestCase, from which the others inherit. It may not be obvious to users that these classes inherit from TransactionTestCase, so I just added a mention of this here.