Consistently document argument and return types using type annotations

[pvandyken]

This is related to #249, and will be partially solved by its completion. Right now we have a mixture of type documentation in the docstring and as type annotations. Sphinx handles type annotations just fine, so we should eliminate all docstring type annotations in there favour. This will help limit code duplication and developer burden (already I think some of the docstring annotations are out of date).

On the same token, we need to explicitly annotate the return type func() -> type: ... for all public functions. This isn't necessary for pyright, but it is necessary for sphinx autodoc. It's also probably a good practice for public endpoints, to make sure we don't accidentally change the return type of functions we don't consume internally (on the chance our test suite doesn't cover them).