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

related to https://code.djangoproject.com/ticket/23278
I removed this change as the other logging blocks could be cleaned up as well and that would be better in a separate commit. For example, we should use `.. code-block:: python` here rather than `::` to make syntax highlighting work (it doesn't work because the block isn't parsed properly without the outer dictionary brackets. Would you like to work on this? I can, if not. It's not very interesting work.