docs($http): clear up Interceptor docs and code

Request Type: docs

How to reproduce: 

Component(s): $http

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

The documentation and code example of $http interceptors is unclear about whether config can be null
or not, and whether the result should always be a promise or not. This pr clears up the documentation
a bit and removes the literal 'or a promise' interpretation of the docs in the code example.

See #7431

**Other Comments:**