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

審查 http.cookiejar 的文件和現有的 PR #135882，重點關注公開方法 CookieJar.clear_expired_cookies。完成的標準是：在不改變該方法行為的情況下，為實作自訂 cookie 原則的使用者撰寫該方法的文件。