Improve the documentation and logging for the HTTP Digest auth backend.
Review Request #9993 — Created May 31, 2018 and submitted
This fleshes out the docs for the HTTP Digest backend, detailing the
function signatures and the Django/siteconfig settings that control it.
It also makes use of a specific logger for any logging, helping provide
context for any errors.
Unit tests pass.
Built the docs and checked them for build/spelling/reference errors.