Added documentation for ajaxError with DS.Errors.

If my findings are right in #2028, then implementing `ajaxError` is required in order for `DS.Errors` to work.
Lets make `ajaxError` here a link to the `RESTAdapter`'s API docs (http://emberjs.com/api/data/classes/DS.RESTAdapter.html#method_ajaxError) because the example used there is a good starting point for making this functionality work with other adapters.