Fixed #23512 -- Made migration docs about initial_data clearer

  it as soon as possible (nothing more than renaming is required).

* ``initial_data`` fixtures are no longer loaded for apps with migrations; if

  you want to load initial data for an app, we suggest you do it in a migration.

  you want to load initial data for an app, we suggest you create a migration for

  your application and define a :class:`~django.db.migrations.operations.RunPython`

  or :class:`~django.db.migrations.operations.RunSQL` operation in the ``operations`` section of the migration.

* Test rollback behavior is different for apps with migrations; in particular,

  Django will no longer emulate rollbacks on non-transactional databases or