`pytest.Instance` is not documented in the API Reference

[bluetech]

The Instance collector type is part of the public API and is passed to some hooks, e.g. pytest_pycollect_makeitem, so we should explain what it is (it is not very obvious!).

[bluetech]

Looking at Instance, I'm think we might want to get rid of it instead of publicizing it more. I'll post a proposal for this after #9273 is merged.

[RonnyPfannschmidt]

@bluetech i want to remove instance for more than 5 years now ^^ and at the same time introduce FunctionDefinition more directly

[bluetech]

This is made irrelevant by #9277.