Fixed #20590: Documented new test case ordering

In order to guarantee that all ``TestCase`` code starts with a clean database,

the Django test runner reorders tests in the following way:

* First, all unittests (including :class:`unittest.TestCase`,

  :class:`~django.test.SimpleTestCase`, :class:`~django.test.TestCase` and

  :class:`~django.test.TransactionTestCase`) are run with no particular ordering

  guaranteed nor enforced among them.

* All :class:`~django.test.TestCase` subclasses are run first.

* Then, all other unittests (including :class:`unittest.TestCase`,

  :class:`~django.test.SimpleTestCase` and

  :class:`~django.test.TransactionTestCase`) are run with no particular

  ordering guaranteed nor enforced among them.

* Then any other tests (e.g. doctests) that may alter the database without

  restoring it to its original state are run.