docs($http) Clarify how to specify JSONP callback.

Request Type: docs

How to reproduce: 

Component(s): $http

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**

Make clear that it is the name of the callback that should be `JSON_CALLBACK`, instead of the current vague description that implies the string can be located anywhere in the URL.