docs($cookies): add info about angular-cookies.js

per the [top comment here](http://docs.angularjs.org/api/ngCookies.$cookies#comment-912064775)

updating documentation so it matches [$resource](http://docs.angularjs.org/api/ngResource.$resource)
and instructs the user to include the `angular-cookies.js` and load `ngCookies`.

Closes #3607