Clean up cookie docstrings, document cookie jar.

Following on from [a Stack Overflow question](https://stackoverflow.com/questions/27586779/requests-inability-to-handle-two-cookies-with-same-name-different-domain), this PR extends our cookie documentation [here](http://docs.python-requests.org/en/latest/api/#cookies) to document the cookie jar itself. This should make it more likely that users are able to discover how to make more subtle queries about cookies.
This first line makes no sense.