Fixed #23349 -- Clarified details about RunPython's apps argument.

If you're upgrading from South, this is basically the South pattern as an

operation - one or two methods for forwards and backwards, with an ORM and

schema operations available. You should be able to translate the ``orm.Model``

or ``orm["appname", "Model"]`` references from South directly into

``apps.get_model("appname", "Model")`` references here and leave most of the

rest of the code unchanged for data migrations.

schema operations available. Most of the time, you should be able to translate

the ``orm.Model`` or ``orm["appname", "Model"]`` references from South directly

into ``apps.get_model("appname", "Model")`` references here and leave most of

the rest of the code unchanged for data migrations. However, ``apps`` will only

have references to models in the current app unless migrations in other apps

are added to the migration's dependencies.

Much like :class:`RunSQL`, ensure that if you change schema inside here you're

either doing it outside the scope of the Django model system (e.g. triggers)