Update docs for `inject` method https://github.com/emberjs/ember.js/issues/3670

Replace `model` with `controller` as an example of `inject` method usage because injection into model is currently disabled. 

Copied a note on injection into model from https://github.com/emberjs/ember.js/blob/master/packages/container/lib/main.js#L13
Merge pull request #3861 from arkadiyk/patch-1

Update docs for `inject` method https://github.com/emberjs/ember.js/issu...