[OptionsResolver] Adjusted the OptionsResolver documentation to describe the 2.6 API

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11716)
| Applies to    | 2.6+
| Fixed tickets | -
No, read on. Everything is written for 2.6, there's no mention of the `OptionsResolver` anymore.