feature symfony#11310 [Validator] Added documentation for Traverse co…

…nstraint (HeahDude)

This PR was squashed before being merged into the 3.4 branch (closes symfony#11310).

Discussion
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[Validator] Added documentation for Traverse constraint

```rst
versionadded:: 2.5

    The traverse constraint has been added in Symfony 2.5
```
x)

Commits
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86ee59e [Validator] Added documentation for Traverse constraint

reference/constraints/Traverse.rst

@@ -0,0 +1,98 @@
+Traverse
+========
+
+Objects do not validate nested objects by default unless explicitly using
+this constraint.
+If only specific nested objects should be validated by cascade, consider
+using the :doc:`references/constraints/Valid` instead.
+
++----------------+-------------------------------------------------------------------------------------+
+| Applies to     | :ref:`class <validation-class-target>`                                              |
++----------------+-------------------------------------------------------------------------------------+
+| Options        | - `payload`_                                                                        |
++----------------+-------------------------------------------------------------------------------------+
+| Class          | :class:`Symfony\\Bridge\\Doctrine\\Validator\\Constraints\\Traverse`                |
++----------------+-------------------------------------------------------------------------------------+
+
+Basic Usage
+-----------
+
+In the following example, create three classes ``Book``, ``Author`` and
+``Editor`` that all have constraints on their properties. Furthermore,
+``Book`` stores an ``Author`` and an ``Editor`` instance that must be
+valid too. Instead of adding the ``Valid`` constraint to both fields,
+configure the ``Traverse`` constraint on the ``Book`` class.
+
+.. configuration-block::
+
+    .. code-block:: php-annotations
+
+        // src/AppBundle/Entity/Book.php
+        namespace AppBundle\Entity;
+
+        use Symfony\Component\Validator\Constraints as Assert;
+        use Doctrine\ORM\Mapping as ORM;
+
+        /**
+         * @ORM\Entity
+         * @Assert\Traverse
+         */
+        class Book
+        {
+            /**
+             * @var Author
+             *
+             * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Author")
+             */
+            protected $author;
+
+            /**
+             * @var Editor
+             *
+             * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Editor")
+             */
+            protected $editor;
+
+            // ...
+        }
+
+    .. code-block:: yaml
+
+        # src/AppBundle/Resources/config/validation.yml
+        AppBundle\Entity\Book:
+            constraints:
+                - Symfony\Component\Validator\Constraints\Traverse: ~
+
+    .. code-block:: xml
+
+        <!-- src/AppBundle/Resources/config/validation.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping https://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">
+
+            <class name="AppBundle\Entity\Book">
+                <constraint name="Symfony\Component\Validator\Constraints\Traverse"/>
+            </class>
+        </constraint-mapping>
+
+    .. code-block:: php
+
+        // src/AppBundle/Entity/Book.php
+        namespace AppBundle\Entity;
+
+        use Symfony\Component\Validator\Constraints as Assert;
+        use Symfony\Component\Validator\Mapping\ClassMetadata;
+
+        class Book
+        {
+            public static function loadValidatorMetadata(ClassMetadata $metadata)
+            {
+                $metadata->addConstraint(new Assert\Traverse());
+            }
+        }
+
+Options
+-------
+
+.. include:: /reference/constraints/_payload-option.rst.inc