[Validator] Added grouped constraints in Collection reference

reference/constraints/Collection.rst

@@ -289,6 +289,40 @@ However, if the ``personal_email`` field does not exist in the array,
 the ``NotBlank`` constraint will still be applied (since it is wrapped in
 ``Required``) and you will receive a constraint violation.
 
+When you define groups in nested constraints they are automatically added to
+the ``Collection`` constraint itself so it can be traversed for all nested
+groups. Take the following example::
+
+    use Symfony\Component\Validator\Constraints as Assert;
+
+    $constraint = new Assert\Collection([
+        'fields' => [
+            'name' => new Assert\NotBlank(['groups' => 'basic']),
+            'email' => new Assert\NotBlank(['groups' => 'contact']),
+        ],
+    ]);
+
+This will result in the following configuration::
+
+    $constraint = new Assert\Collection([
+        'fields' => [
+            'name' => new Assert\Required([
+                'constraints' => new Assert\NotBlank(['groups' => 'basic']),
+                'groups' => ['basic', 'strict'],
+            ]),
+            'email' => new Assert\Required([
+                "constraints" => new Assert\NotBlank(['groups' => 'contact']),
+                'groups' => ['basic', 'strict'],
+            ]),
+        ],
+        'groups' => ['basic', 'strict'],
+    ]);
+
+The default ``allowMissingFields`` option requires the fields in all groups.
+So when validating in ``contact`` group, ``$name`` can be empty but the key is
+still required. If this is not the intended behavior, use the ``Optional``
+constraint explicitly instead of ``Required``.
+
 Options
 -------
 

validation/raw_values.rst

@@ -105,3 +105,10 @@ The ``validate()`` method returns a :class:`Symfony\\Component\\Validator\\Const
 object, which acts like an array of errors. Each error in the collection
 is a :class:`Symfony\\Component\\Validator\\ConstraintViolation` object,
 which holds the error message on its ``getMessage()`` method.
+
+.. note::
+
+    When using groups with the
+    :doc:`Collection</reference/constraints/Collection>` constraint, be sure to
+    use the ``Optional`` constraint when appropriate as explained in its
+    reference documentation.