[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 | -
but the sentence saying to use the version switcher in the upper right is not consistent at all with the way the doc is written.

Thus, I thinkk the OptionsResolver should still be mentionned somewhere, otherwise people using the Form component will get confused (and it is probably the first place where Symfony users will meet the OptionsResolver component)