[DOC Beta] New behaviour with itemController

This [BUGFIX beta - Ensure context is unchanged when using keywords with itemController](https://github.com/emberjs/ember.js/pull/4636) changed the way `itemController` behaves inside `#each` helper. I had problems making it work out because documentation doesn't clear this out and I had to debug it.

These two JsBins describe what changed from the developer using `#each` helper with `itemController`.

- [JsBin with Ember (1.5.1)](http://jsbin.com/ucanam/4838/edit) -> we can call itemController's property without prepending `person`

- [JsBin with Ember (1.6.0-beta.3)](http://jsbin.com/ucanam/4837/edit) -> if we don't prepend `person` for itemController's property we won't get result