docs(ngResource): clarify behaviour of $promise

Closes angular#6753

Author: alexsanford

src/ngResource/resource.js

@@ -199,6 +199,9 @@ function shallowClearAndCopy(src, dst) {
  *     On failure, the promise is resolved with the {@link ng.$http http response} object, without
  *     the `resource` property.
  *
+ *     If an interceptor object was provided, the promise will instead be resolved with the value
+ *     returned by the interceptor.
+ *
  *   - `$resolved`: `true` after first server interaction is completed (either with success or
  *      rejection), `false` before that. Knowing if the Resource has been resolved is useful in
  *      data-binding.