docs($http): add better explanation for error status codes
docs($http): add better explanation for error status codes

Add explanation for error status codes in `$http` and mention negative codes normalization.
This alleviates confusion about which status codes will trigger the error callback and what values
to expect in such callbacks.

Fixes #11945

Closes #14459

Real-Currents/angular.js,Real-Currents/angular.js,Real-Currents/angular.js,Real-Currents/angular.js