docs: clarify typing for yield fixtures

[phalberg]

Description

While working with adding types to a codebase of mine and using mypy it was not so trivial (for me at least) that fixtures that are of Generator types, are not to be type annotated like Generators but rather their return types when used in other fixtures. Based on: https://docs.pytest.org/en/stable/how-to/fixtures.html#teardown-cleanup-aka-fixture-finalization, it became more obvious why this is the case, but it was not so easy to find this documentation, and I feel that the typing docs can also have a note for it.

Changes

Added a typing section about usage of yield based fixtures for other fixtures.

Relevant issue

No relevant issue, as I see this as small docs "enhancement".

[phalberg]

I could also include an example of iterator for this too? If this is not already clear from this perhaps... or at least just mention it maybe