Fixed #23860 -- Document imports order convention.

Please see [#23860](https://code.djangoproject.com/ticket/23860).
Nitpick: The examples are a mix of Python 2 and Python 3 stdlib. `nested_scopes` is too old. I'd replace it with something like `print_function`. Also, `concurrent.futures` is only available on Python 3 stdlib. I'd replace it with `email.message`.