Fixed #23278 - Add documentation for django.db.backends.schema logger

related to https://code.djangoproject.com/ticket/23278
This looks like it's referring to `extra` kwarg, e.g.
```
logger.warning('Not Found: %s', request.path,
                        extra={
                            'status_code': 404,
                            'request': request
                        })
```
So I don't think the schema logger has any of that.