Add description for the "validation_groups" option

[webmozart]

The documentation of the "validation_groups" option for the "form" type is missing. The option should make clear what the options actually does, which seems to be confusing for many users. See symfony/symfony#12342 for example.

[wouterj]

To be clear: This has to be added to /reference/form/types/form.rst.

The option is only valid on the root node and is used to specify which groups will be used by the validator. This should be made clear in the description.

[xabbuh]

Additionally, we should add the same warning (that this option is only evaluated for the root form type) in the book where we talk about validation groups.

[linaori]

It's slightly documented on: http://symfony.com/doc/current/form/validation_groups.html

However, it's missing the callback documentation and it's not listed on https://symfony.com/doc/current/reference/forms/types/form.html