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

[garetht]

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.

[mary-poppins]

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[mary-poppins]

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