Make it easy to use types from `_typeshed` in docstrings

[lagru]

Certain types are only available via Python's _typeshed module, e.g., DataclassInstance. It would be nice if you could use these out of the box in Docstrings.

Adding this feature will involve figuring out how to get a list the types in the _typeshed into docstub. One problem is, that they aren't available at runtime...