multiple recipients exists but isn't referenced in the documentation

[hex333ham]

it can actually be enabled by adding the following to the fos_message config in services.yaml:

    new_thread_form:
      type:               FOS\MessageBundle\FormType\NewThreadMultipleMessageFormType
      handler:            fos_message.new_thread_multiple_form.handler
      model:              FOS\MessageBundle\FormModel\NewThreadMultipleMessage
      name: message

looks experimental, needs revisiting at some point