gh-101100: Fix Sphinx warnings in c-api/structures.rst

[hugovk]

Fix these two warnings:

Doc/c-api/structures.rst:519: WARNING: c:type reference target not found: PyTypeSlot Doc/c-api/structures.rst:524: WARNING: c:type reference target not found: PyTypeSlot 

They're typos, should be PyType_Slot. See PR #103511 where they were added (to 3.12, no backport needed for 3.11).

That leaves these two warnings in this file:

Doc/c-api/structures.rst:656: WARNING: c:identifier reference target not found: getter Doc/c-api/structures.rst:660: WARNING: c:identifier reference target not found: setter 

I'm not sure what to do with these, any ideas? (Added in PR #100054, also in 3.12.)

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--113564.org.readthedocs.build/

[hugovk]

@encukou Any suggestion for the getter/setter warnings here?

[encukou]

They're defined a few lines below. (Definitely not the first place I looked!)

I hope you're OK with me pushing a fix-up directly to the PR.
I also found a nearby improvement to make: for struct fields, NULL works better than “omitted”.

[hugovk]

I am, thank you!

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-114527 is a backport of this pull request to the 3.12 branch.