[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 | -
>
The getSupportedAttributes method tells Symfony that your voter should be called whenever one of these strings is passes as the first argument to `isGranted`. For example, if you return `array('CREATE', 'READ')`, then Symfony will call your voter when one of these is passed to `isGranted`.