Docs: fill in descriptor C API docs

[Boyeep]

This documents several missing entries in Doc/c-api/descriptor.rst.

Changes in this PR:

• document PyDescr_NewGetSet
• document PyDescr_NewMember
• document PyDescr_NewMethod
• document PyDescr_NewWrapper
• document PyDescr_NewClassMethod
• document PyWrapper_New
• fix nearby Python type references for descriptor objects so they match the actual types module names

This is a docs-only change.

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📚 Documentation preview 📚: https://cpython-previews--146644.org.readthedocs.build/

[python-cla-bot]

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[ZeroIntensity]

Thanks for tackling these.

[ZeroIntensity]

wrapperbase is undocumented as well -- can we fix that in this PR?

[Boyeep]

Sure, I'll be working on it

[ZeroIntensity]

Let's elaborate a little more (the same goes for the ones below):

1. What is a get-set descriptor, exactly?
2. Why do users care? (Or, what might they use this for?)
3. What does this correspond to in Python?

[Boyeep]

Implemented, thanks! I expanded the new descriptor docs to explain:

1. what each descriptor is
2. where extension authors encounter it (tp_getset, tp_members, tp_methods, and slot-based special methods)
3. why it is useful
4. and what it corresponds to in Python.

I also documented wrapperbase, which fixes the docs warning for that identifier.

[Boyeep]

Added a follow-up to remove Doc/c-api/descriptor.rst from Doc/tools/.nitignore now that the new references are resolved and the page builds cleanly in nitpicky mode. That should address the Docs / Docs CI failure from the previous push.