mark deep option as deprecated

Author: Henry Snoek

reference/constraints/Valid.rst

@@ -9,7 +9,7 @@ an object and all sub-objects associated with it.
 | Applies to     | :ref:`property or method <validation-property-target>`              |
 +----------------+---------------------------------------------------------------------+
 | Options        | - `traverse`_                                                       |
-|                | - `deep`_                                                           |
+|                | - `deep`_ (deprecated as of 2.5)                                    |
 |                | - `payload`_                                                        |
 +----------------+---------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\Valid`          |
@@ -271,6 +271,12 @@ set to ``true``.
 deep
 ~~~~
 
+.. caution::
+
+    The ``deep`` option was deprecated in Symfony 2.5 and will be removed
+    in Symfony 3.0. When traversing arrays, nested arrays are always traversed.
+    When traversing nested objects, their traversal strategy is used.
+
 **type**: ``boolean`` **default**: ``false``
 
 If this constraint is applied to a property that holds an array of objects,