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

审查 http.cookiejar 的文档和现有的 PR #135882，重点关注公共方法 CookieJar.clear_expired_cookies。完成的标准是：在不改变该方法行为的情况下，为实现自定义 cookie 策略的用户编写该方法的文档。