Document SimpleTestCase.async_client_class

[Adam Johnson]

We currently document the sync version, client_class: ​https://docs.djangoproject.com/en/6.0/topics/testing/tools/#django.test.SimpleTestCase.client_class . But since async_client_class is not documented, there’s no clear way to swap your async test client class, unless you read the source. Let’s fix that by documenting async_client_class equally.