The warnings module makes use of a per-module global ``__warningregister__`` which is briefly mentioned in the docs but not documented.

https://docs.python.org/3/library/warnings.html#warnings.warn_explicit

Given that it is part of the warn_explicit API, we should accept that the registry file is now part of the public API not just an implementation detail, and so we should explicitly document its existence and state the expected format.