choice_value callback argument can be null

[xabbuh]

For example, when the initial data of the form type is null, the callback used for the choice_value option will receive null as an argument. We need to make this clear in the reference of the ChoiceType. See symfony/symfony#21116 for more context information.

[HeahDude]

There were also symfony/symfony#20402, symfony/symfony#19246.

One of my resolution for 2017 is to get back to work on the docs :), so a first step will be to finish #6393.