docs($http): Add explanation for error status

[AbhishekGarg]

Add explanation for error status codes in $http call and line about normalization to zero in case of negative status code.
This alleviates confusion about status codes for which error callback would be called and final values to be expected from an erroneous call.

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[gkalpak]

@AbhishekGarg, we need a signed CLA before we can merge this.
Also, please add Fixes #11945 at the end of your commit message, so that issue it closed when/if this is merged.

[AbhishekGarg]

I have signed the CLA.
@gkalpak I'll make the suggested changes and changes for successful build asap.

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[AbhishekGarg]

@gkalpak Updated commit message and changed for travis-ci check.
Could you please have a look?

[gkalpak]

Only codes < -1 are normalized. -1 is returned as is (and usually, means aborted, e.g. via timeout).

[gkalpak]

I tweaked it a bit and merged. Thx !