bpo-43225: [DOC] Add missing value returned by methods on cookiejar (GH-24522)

bpo-43225: [DOC] Add missing value returned by methods on cookiejar (GH-24522)

* Add missing value returned by methods on cookiejar

Documentation say that return something but don't specific with what
value is return, and that can be confuse. This patch add that information.

* Update Doc/library/http.cookiejar.rst

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>
* Update Doc/library/http.cookiejar.rst

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>
Co-authored-by: andrei kulakov <andrei.avk@gmail.com>