Refs #25251: update settings.SERIALIZE description related to --keepdb issue.

[romgar]

Using SERIALIZE = False prevents tests with serialize_rollback = True to work as expected, and is described in the documentation.
But it will also prevent the --keepdb option to properly keep the serialized initia data at the end of the test suite.
This PR is updating the documentation according to this, and is related to some discussions with @timgraham in #6297.

[timgraham]

Do you think this is a bit more clear?

If you have data migrations and use :class:`~django.test.TransactionTestCase`
along with the :option:`test --keepdb` option, you won't see data from
migrations in subsequent ``--keepdb`` test runs if you set ``SERIALIZE`` to
``False``.

[romgar]

@timgraham That's definitely clearer.

I'm just wondering if it makes sense at the end to update this documentation on a separate PR than #6297.
Indeed, users won't see data migrations in subsequent --keepdb test runs if they set SERIALIZE to either True or False (until we fix https://code.djangoproject.com/ticket/25251), which is a bit misleading when we read the doc update in this PR.

[timgraham]

Sure, you can put this with that PR.