feature #4348 Updated information about handling validation of embedded forms to Valid... (peterrehm)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4348).

Discussion
----------

Updated information about handling validation of embedded forms to Valid...

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4346

Applied Valid constraint instead of the cascade_validation option since this option is supposed to be deprecated with symfony/symfony#12237

Commits
-------

b699731 Updated information about handling validation of embedded forms to Valid constraint

Author: weaverryan

book/forms.rst

@@ -1234,6 +1234,8 @@ objects. For example, a registration form may contain data belonging to
 a ``User`` object as well as many ``Address`` objects. Fortunately, this
 is easy and natural with the Form component.
 
+.. _forms-embedding-single-object:
+
 Embedding a Single Object
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1263,6 +1265,7 @@ Next, add a new ``category`` property to the ``Task`` class::
 
         /**
          * @Assert\Type(type="Acme\TaskBundle\Entity\Category")
+         * @Assert\Valid()
          */
         protected $category;
 
@@ -1279,6 +1282,12 @@ Next, add a new ``category`` property to the ``Task`` class::
         }
     }
 
+.. tip::
+
+    The ``Valid`` Constraint has been added to the property ``category``. This
+    cascades the validation to the corresponding entity. If you omit this constraint
+    the child entity would not be validated.
+
 Now that your application has been updated to reflect the new requirements,
 create a form class so that a ``Category`` object can be modified by the user::
 
@@ -1326,16 +1335,7 @@ class:
     }
 
 The fields from ``CategoryType`` can now be rendered alongside those from
-the ``TaskType`` class. To activate validation on CategoryType, add
-the ``cascade_validation`` option to ``TaskType``::
-
-    public function setDefaultOptions(OptionsResolverInterface $resolver)
-    {
-        $resolver->setDefaults(array(
-            'data_class' => 'Acme\TaskBundle\Entity\Task',
-            'cascade_validation' => true,
-        ));
-    }
+the ``TaskType`` class.
 
 Render the ``Category`` fields in the same way as the original ``Task`` fields:
 

reference/forms/types/options/cascade_validation.rst.inc

@@ -8,8 +8,13 @@ For example, if you have a ``ProductType`` with an embedded ``CategoryType``,
 setting ``cascade_validation`` to ``true`` on ``ProductType`` will cause
 the data from ``CategoryType`` to also be validated.
 
-Instead of using this option, you can also use the ``Valid`` constraint in
-your model to force validation on a child object stored on a property.
+.. tip::
+
+    Instead of using this option, it is recommended that you use the ``Valid``
+    constraint in your model to force validation on a child object stored on
+    a property. This cascades only the validation but not the use of the
+    ``validation_group`` option on child forms. You can read more about this
+    in the section about :ref:`Embedding a Single Object <forms-embedding-single-object>`.
 
 .. include:: /reference/forms/types/options/_error_bubbling_hint.rst.inc