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.

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-135885: Fix undocumented method CookieJar.clear_expired_cookies in lib http.cookiejar #135882