DOC: Add test runner limitation wrt asdf-astropy to Known Issues

[pllim]

Description

This pull request is to address the following:

• close Running tests from astropy.test() fails when asdf-astropy is installed #16165

The only thing that matters here is the RTD build, so I skipped other CI.

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[github-actions]

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[pllim]

pre-commit.ci autofix

[WilliamJamieson]

This seems like a reasonable compromise for now, especially if we move forward with #16177.

[pllim]

Thanks for the review!