One general comment on this section: I think it needs to be made a bit more explicit that generally, `ConfigNamespace`s only live in package `__init__.py` files.  That seems to be your intent here, but the description in the docs doesn't make that clear (probably because it's partly based on the old system which scattered them everywhere).  So maybe just say that near the beginning, or in the first example where you describe `ConfigNamespace`.?