[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 agree that this reads better, though the incorrect method is referenced and it would be "themselves" instead of "themself". It would read as follows:

    Sometimes you want to present the default answer to the user that will be
    chosen if they don't provide an answer themselves. Use the
    :method:`Symfony\\Component\\Console\\Question\\Question::setShowDefault` method
    to show the default answer::

Let me know and i'll commit this.