I guess the use case isn't quite intuitive and I'll need to fix that.
For the normal case, you don't need to bother with calling this at all as the initializer takes care of everything.
For the standard test case (or manual use), register_single is the way to go - together with disabling automatic_load.
But for the case that you may want to test registering multiple documents, I wanted to keep the register and dereference_queued_descriptor_hashes_and_build_registry accessible.

This would not be the regular use case!