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.
Not your fault, but this should be double backticks around requests.