[WCM] Added note about ProgressBar changes

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | symfony/symfony#11852
| Applies to    | 2.6
| Fixed tickets | n/a
I would add an API link for the method:

```rst
:method:`redraw frequency <Symfony\Component\Console\Helper\ProgressBar::setRedrawFrequency>`
```