gh-101100: Fix sphinx warnings in library/asyncio-dev.rst

[sobolevn]

Please, note that this change is not about asyncio-eventloop.rst, but is about asyncio-dev.rst which just happens to reference asyncio-eventloop.rst:

/Users/sobolev/Desktop/cpython/Doc/library/asyncio-dev.rst:60: WARNING: py:attr reference target not found: loop.slow_callback_duration 

I've also fixed one asyncio-eventloop.rst problem, which cannot be detected by Sphinx (just because I saw it):

I've moved the .. versionchanged inside the function body.

In the next PR I will fix warnings specific to asyncio-eventloop.rst.

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

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📚 Documentation preview 📚: https://cpython-previews--111179.org.readthedocs.build/

[willingc]

Thanks @sobolevn for the PR. I've added a small clarification to the attribute definition.

[miss-islington-app]

Thanks @sobolevn for the PR, and @willingc for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-111185 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-111186 is a backport of this pull request to the 3.11 branch.