[2.8] Document new form features

[althaus]

As a followup of #5179: The news list new features in the form component. Would be great to update the reference accordingly:

• Added the prototype_data option to collection type
• Added choice_translation_domain option for date-related types (this is missing in general for 2.7)
• Allowed to return null for query_builder
• Foundation 5 form theme

Ping @weaverryan

Cheers
Matthias

[weaverryan]

Thanks @althaus - I've marked this for 2.8 (as most of it is relevant to 2.8) and marked it as actionable.

[althaus]

OK. Will check if I can do some basics on this one again.

[wouterj]

Fyi, I've check (4) as it's now mentioned in http://symfony.com/doc/current/cookbook/form/form_customization#what-are-form-themes The others seem to be still valid. Please note that the choice_translation_domain has now documentation, so it's simply a case of including the option in the date related articles.

[HeahDude]

Two first boxes can be checked now :)

[wouterj]

The last open points is about http://symfony.com/doc/current/reference/forms/types/entity#query-builder It should be documented to accept returning null. See http://symfony.com/blog/new-in-symfony-2-8-form-improvements#allowed-to-return-null-for-query-builder for more details.

It should be kept short and straight-to-the-point, like "You can return null to achieve X".

[JonEastman]

@wouterj I'll give the last point a shot!