docs($httpBackend): updated response data api

[robinj]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Docs update

What is the current behavior? (You can also link to an open issue here)
Missing return type in documentation for httpBackend requestHandler function

What is the new behavior (if this is a feature change)?
Added missing return type in documentation

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

Updated docs to reflect that response data can either be an object or a string

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[robinj]

I signed it!

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