[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 | -
This example should actually depend on another option (which the title says). Otherwise one could argue whether is a lazy option or a normalizer. So I would maybe change the example to
```
class Mailer
{
    // ...
    protected function setDefaultOptions(Options $options)
    {
        // ...
        $options['encryption'] = null;
        $options['host'] = 'example.org';
    }
}

class GoogleMailer extends Mailer
{
    protected function setDefaultOptions(Options $options)
    {
        parent::setDefaultOptions($options);

        $options['host'] = function (Options $options, $previousValue) {
            if ('ssl' === $options['encryption']) {
                return 'secure.example.org'
            }
             
            return $previousValue;
        };
    }
}
```