Add an `autodoc_typehints_return` option

Hi there,

I think it would be cool to explicitly tell autodoc not to document the return type automatically when autodoc_typehints in {"description", "both"}.

Indeed, it is generally ugly when documenting a tuple return type such as the following.

def foo(): -> tuple[int, int]:
    """Foooo.

    Returns
    -------
    a: int
        Bar.
    b: int
        Baz.
    """
    return 0, 1

Note that the napoleon option napoleon_use_rtype = False does not work, and also, one cannot use the autodoc-process-docstring event since the return type specification seems to be added after it...

Options could be "all", "none", "undocumented" for example.

All the best!
Élie.

Start by tracing autodoc_typehints and the autodoc-process-docstring event, checking where return types are added relative to that event; napoleon_use_rtype is noted as ineffective here. Define the option semantics, such as all, none, or undocumented, and verify that tuple-return documentation does not duplicate documented fields while other return types follow the selected behavior.