gh-96471: Correct documentation for asyncio queue shutdown

[EpicWink]

Fix documentation of behaviour introduced by #104228

• Gets are always unblocked (see gh-96471: Add asyncio queue shutdown #104228 (comment))
• Add comment for reason why gets are always unblocked
• Acknowledge Yves D (see gh-96471: Add asyncio queue shutdown #104228 (comment))

• Issue: Queue shutdown #96471

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📚 Documentation preview 📚: https://cpython-previews--117621.org.readthedocs.build/

[YvesDup]

I approved.
@EpicWink , @gvanrossum I am very happy to have succeeded in these PRs together