Document $q's notify/progress behaviour

[JensRantil]

I just noticed that the #2223 did not document the introduction of progress notification. I guess this should be done somewhere in https://github.com/angular/angular.js/blob/master/src/ng/q.js.

The tests in https://github.com/angular/angular.js/pull/2223/files#diff-1 could be a good starting point to understand the concept.