feature symfony#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 symfony#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 | symfony#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