[WCM] Question Helper - Showing the default answer

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes - symfony/symfony#11975
| Applies to    | 2.5
| Fixed tickets | 

This is the documentation for a new feature that I am proposing.
This should be "Showing the Default Answer" (only closed class words are not capititalized in our headlines)