IMHO you aren’t thinking of this right and you kinda hit the nail on the head with this statement.
Everything you stated is correct, nonetheless where is the need to serve FooType.qux(_:). The documentation for FooModule shouldn’t worry about including documentation for symbols that don’t exist yet. Furthermore, if FooType is just using the default implementation provided by QuxModule then users should refer to QuxModule’s documentation. If an imaginary package provides a custom implementation of FooType.qux(_:) then it should be the one documenting it, but that ties into the excellent work @theMomax has been doing as part of Document Extensions to External Types Using DocC,