Document that RunPython callables receive a StateApps instance

[Thibaut Decombe]

The ​`RunPython` docs say the first argument passed to code / reverse_code is "an instance of django.apps.registry.Apps containing historical models".
At runtime it is a django.db.migrations.state.StateApps (see django/db/migrations/operations/special.py, ​RunPython.database_forwards().

This difference matters to users who follow the docs literally, especially when annotating migration functions:

def forwards(apps: Apps, schema_editor: BaseDatabaseSchemaEditor) -> None:
    Person = apps.get_model("myapp", "Person")

Apps is also the type of the global registry (django.apps.apps), whose get_model() returns the current models from models.py.

With the documented annotation, tools such as the django-stubs mypy plugin can't tell this historical registry apart from the global one. They resolve get_model() to the current model class, which may have different fields than the historical model (​https://github.com/typeddjango/django-stubs/issues/3584).

Proposal: mention django.db.migrations.state.StateApps (as a subclass of Apps) in the RunPython docs.