[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 think we should actually use the AbstractVoter in this example. Other than mentioning that VoterInterface exists and you can just implement it, I don't see any disadvantage to always using AbstractVoter. Agree?