[WCM] Question Helper - Showing the default answer

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes - symfony/symfony#11975
| Applies to    | 2.6
| Fixed tickets | 

This is the documentation for a new feature that I am proposing.
I think we should reword this a bit to something like this:

```rst
Sometimes you want to present the default answer to the user that will be
chosen if they don't provide an answer themself. Use the
:method:`Symfony\\Component\\Console\\Question\\Question::setDefault` method
to show the default answer::
```

What do you think?