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.
Switch `Caution: ` to `.. warning::` here too?