Fix undocumented method `CookieJar.clear_expired_cookies` in lib `http.cookiejar`

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.

IMO we can document it in case of ppl are customizing their own policy instead of using DefaultCookiePolicy, while they need this func to write their own expiration logic.

PR: #135882

Linked PRs

• gh-135882

http.cookiejar のドキュメントと既存の PR #135882 を確認し、公開メソッド CookieJar.clear_expired_cookies に焦点を当てます。ユーザーがカスタム Cookie ポリシーを実装するためにこのメソッドを利用できるよう、その動作を変更せずにドキュメント化されていれば完了です。