[Security] add & update doc entries on AbstractVoter implementation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (related to https://github.com/symfony/symfony/pull/11183)
| Applies to    | 2.6
| Fixed tickets | -
I'm skipping all formatting, but what about:
>
The getSupportedClasses method tells Symfony that your voter should be called whenever an object of one of the given classes is returned. For example, if you return `array('\Acme\DemoBundle\Model\Product')`, Symfony will call your voter when a ``Product`` object is passed to ``isGranted()``.