Clarify section on view controllers

Relating to [laravel/framework issue #1822](https://github.com/laravel/framework/issues/1822): I found the section of the docs on view controllers a bit misleading. After digging into the source I found that the composers are invoked on *render* (see [Illuminate\View\View line 79](https://github.com/laravel/framework/blob/3ed3f4eb995edce02bbfa4f6de50ba122d852e39/src/Illuminate/View/View.php#L79)) and not on creation [as the docs currently state](http://laravel.com/docs/responses#view-composers). So I fixed the wording (and clarified exactly when the composers are called) and added some documentation for registering controllers like controller methods: `ViewController@someComposeMethod`.