minor #7754 Adding a tip for validation in forms without class (stoccc, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7754).

Discussion
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Adding a tip for validation in forms without class

When a form is mapped to an array instead of an object (i.e. a search form), every object in the array is validated with Symfony\Component\Validator\Constraints\Valid constraint.

For example a search form mapped to an array including a field of type Symfony\Bridge\Doctrine\Form\Type\EntityType will validate the choosen entity itself, resulting in an invalid form if the (already persisted) entity is, for some reasons, invalid.

References: https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php#L171:L186

I'm wondering if this behavior is a bug, while the validation of objects has this approach
"If an object is passed without explicit constraints, validate that object against the constraints defined for the object's class [eventually none]"
https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php#L154:L169

Commits
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eb26f28 Minor reword
3177c1d Adding a tip for validation in forms without class

Author: xabbuh

form/without_class.rst

@@ -105,3 +105,9 @@ but here's a short example:
     .. code-block:: php
 
         new NotBlank(array('groups' => array('create', 'update')));
+
+.. tip::
+
+    If the form is not mapped to an object, every object in your array of
+    submitted data is validated using the ``Symfony\Component\Validator\Constraints\Valid``
+    constraint, unless you :doc:`disable validation </form/disabling_validation>`.