[Form] defined default and added example CollectionType prototype_data option

Author: HeahDude

reference/forms/types/collection.rst

@@ -367,10 +367,28 @@ well as :ref:`cookbook-form-collections-new-prototype`.
 prototype_data
 ~~~~~~~~~~~~~~
 
+.. versionadded:: 2.8
+ The ``prototype_data`` option was introduced in Symfony 2.8.
+
 **type**: ``mixed`` **default**: ``null``
 
-Allows you to specify data for the prototype. Each new row added will initially
-contain the data set by this option.
+Allows you to define specific data for the prototype. Each new row added will
+initially contain the data set by this option. By default, the data configured
+for all entries with the ``entry_options`` option will be used.
+
+.. code-block:: php
+
+ use Symfony\Component\Form\Extension\Core\Type\CollectionType;
+ use Symfony\Component\Form\Extension\Core\Type\TextType;
+ // ...
+
+ $builder->add('tags', CollectionType::class, array(
+ 'class' => 'AppBundle\Entity\Tag',
+ 'entry_type' => TextType::class,
+ 'allow_add' => true,
+ 'prototype' => true,
+ 'prototype_data' => 'New Tag Placeholder',
+ ));
 
 prototype_name
 ~~~~~~~~~~~~~~