gh-90190: Adds doc for singledispatch with GenericAlias annotation

[Delengowski]

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@AlexWaygood

• Issue: gh-90190

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📚 Documentation preview 📚: https://cpython-previews--116544.org.readthedocs.build/

[cpython-cla-bot[bot]]

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[Delengowski]

@savannahostrowski thank you for the review. I believe I resolved all comments.

@AlexWaygood Would you be able to look at this when you get a chance? You originally told me to add you when I mentioned a desire to do this in the linked issue.

[AlexWaygood]

Sorry, this dropped off my radar; will try to take a look soon!

[savannahostrowski]

@AlexWaygood I was going through PRs and noticed that this may have slipped through the cracks. Thoughts?

[miss-islington-app[bot]]

Thanks @Delengowski for the PR, and @AlexWaygood for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12, 3.13. 🐍🍒⛏🤖

[bedevere-app[bot]]

GH-124710 is a backport of this pull request to the 3.13 branch.

[bedevere-app[bot]]

GH-124711 is a backport of this pull request to the 3.12 branch.