Clean up and document the auth backend registry code.
Review Request #9991 — Created May 31, 2018 and submitted
This change improves the documentation for the auth backend registration
functions, meeting modern standards and capabilities.
In the process,
set_enabled_auth_backend()has been removed. It hasn't
been used since pre-1.0, and wasn't worth keeping around. We can safely
remove this and the forwarding import.
Unit tests pass.
Built the docs and checked through them.