Documented the overridden options of "date" type

Author: javiereguiluz

reference/forms/types/date.rst

@@ -30,6 +30,8 @@ day and year) or three select boxes (see the `widget`_ option).
 |                      | - `years`_                                                                  |
 +----------------------+-----------------------------------------------------------------------------+
 | Overridden Options   | - `by_reference`_                                                           |
+|                      | - `compound`_                                                               |
+|                      | - `data_class`_                                                             |
 |                      | - `error_bubbling`_                                                         |
 +----------------------+-----------------------------------------------------------------------------+
 | Inherited            | - `data`_                                                                   |
@@ -125,6 +127,17 @@ by_reference
 
 The ``DateTime`` classes are treated as immutable objects.
 
+.. include:: /reference/forms/types/options/compound_type.rst.inc
+
+data_class
+~~~~~~~~~~
+
+**default**: ``null``
+
+The internal normalized representation of this type is an array, not a ``\DateTime``
+object. Therefore, the ``data_class`` option is initialized to ``null`` to avoid
+the ``FormType`` object from initializing it to ``\DateTime``.
+
 error_bubbling
 ~~~~~~~~~~~~~~