[pylint] Extend docs and test in invalid-str-return-type (E307)

[boolean-light]

Summary

Added some docs, and a little of test cases in invalid-str-return-type, mentioned in #10377 (review)

Test Plan

On invalid_return_type_str.py.

[github-actions]

ruff-ecosystem results

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Linter (preview)

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Formatter (preview)

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[dhruvmanila]

Thanks!