[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 | -
Do we need to import this `TokenInterface` class when using the new `AbstractVoter`?