Document why Django makes its deprecation warnings loud by default and how to silence them

[Tim Graham]

See #18985 for the background of why we route warnings through logging.

A developer can selectively silence warnings using something like this in an app's AppConfig.ready() method:

import warnings
from django.utils.deprecation import RemovedInNextVersionWarning
warnings.simplefilter("ignore", RemovedInNextVersionWarning)

This example can be improved to show how to silence a particular warning instead of all warnings.

This works because AppConfig.ready() methods are called after django.utils.log.configure_logging() (which does warnings.simplefilter("default", RemovedInNextVersionWarning)) in django.setup().