[Form] Document usage of empty_data

[webmozart]

The usage of the empty_data option for forms should be documented. empty_data allows to set the data set in the form when it is bound and setData() wasn't called before.

By default, this is null or, if data_class is set, a new instance of that given class (created by using a no-args constructor).

The option can be overridden by setting a static value (1) or by setting a closure (2), that receives a FormInterface instance as first argument. (2) should be preferred, as it only creates objects on demand.

(1)

public function getDefaultOptions()
{
    return array(
        'empty_data' => new User($this->someDependency)
    );
}

(2)

public function getDefaultOptions()
{
    return array(
        'empty_data' => function (FormInterface $form) {
            return new User($form->get('username')->getData());
        }
    );
}

[webmozart]

Fixes symfony/symfony#1140

[weaverryan]

I think there are two things we can do:

1. Improve the empty_data.rst.inc document with a better example (preferably one using an object, like in Bernhard's example).

2. Create a very short cook book article call "How to control the empty value for a form field" which talks about this. Bernhard's examples are great, so we can just build off of those.