Added shortcut methods for controllers

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11593)
| Applies to    | 2.6+
| Fixed tickets | #4666 

This commit is associated to https://github.com/symfony/symfony/pull/11593 that adds new shortcut methods to controllers.

If anything is wrong with my commit or this description, please tell me and I will fix it as soon as possible. I'm glad I finally try to help this awesome project.

I think we should show both, the second one commented out. So, something like:

```php
public function indexAction()
{
    // redirectToRoute is equivalent to using redirect() and generateUrl() together:
    // return $this->redirect($this->generateUrl('homepage'), 301);

    return $this->redirectToRoute('homepage', 301);
}
```

And then we can add one more thing right below this code block

```rst
Or, if you want to redirect externally, just use ``redirect()`` and pass it the URL::

    public function indexAction()
    {
        return $this->redirect('http://symfony.com/doc');
    }
```