[Security] More accurate docblocks for AbstractVoter

| Q             | A
| ------------- | ---
| Fixed tickets | none
| License       | MIT

Two fixes for docblocks in AbstractVoter:

1. Previously the example return value for a few methods used PHP's short array syntax (>= PHP 5.4). Symfony minimum PHP is 5.3 so the docblock examples should be compatible with that.

2. Previously getSupportedClasses() docblock had an example return value showing a FQCN using a leading slash. AbstractVoter looks for `get_class($object)` in the array of getSupportedClasses() returned values, and `get_class()` returns the FQCN without a leading slash.