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

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11716, symfony/symfony#12156)
| Applies to    | 2.6+
| Fixed tickets | -
The language to highlight is missing:

```rst
.. code-block:: php

    $resolver->setAllowedTypes(...);
```