Complete concatenated properties doc

Documenting the feature of adding a single property should just add it into the related array
Merge pull request #1710 from sly7-7/patch-1

Complete concatenated properties doc
Merge pull request #1707 from tchak/container-as-module

make container a module
ArrayController learned itemControllerClass.

```
var ControllerClass = Em.Controller.extend();
var controller = Em.ArrayController.create({ content: Em.A([0, 1]) });

controller.objectAtContent(0);               // an instance of ControllerClass
controller.objectAtContent(0).get('content') // 0
```

This partially addresses #1637, but makes no attempt at supporting reusable
views.