gh-135885: Fix undocumented method CookieJar.clear_expired_cookies in lib http.cookiejar

[LamentXU123]

Although there is low possibility to call this method if you are using DefaultCookiePolicy, but as a method of CookieJar that could be used(do not start with _, and would be used when users are customizing their own policies), the method is supposed to be documented.

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📚 Documentation preview 📚: https://cpython-previews--135882.org.readthedocs.build/

• Issue: Fix undocumented method CookieJar.clear_expired_cookies in lib http.cookiejar #135885

[LamentXU123]

sorry for clinking the wrong button.

[picnixz]

Pleae open an issue. Adding docs should still have an issue.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[LamentXU123]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.