Clarify behaviour of $promise in ngResource when using an interceptor

Request Type: docs

How to reproduce: 

Component(s): ngResource

Impact: 

Complexity: 

This issue is related to: 

**Detailed Description:**

When an interceptor is used in ngResource, the $promise is no longer resolved with the resource object, but instead with the value returned by the interceptor. This is now specified in the docs.

**Other Comments:**