caution on null values in Expression constraint

This adds a warning to the reference of the Expression constraint
explaining that `null` values won't be validated in Symfony 2.4 and
Symfony 2.5.

Author: xabbuh

reference/constraints/Expression.rst

@@ -128,6 +128,12 @@ One way to accomplish this is with the Expression constraint:
             // ...
         }
 
+.. caution::
+
+    In Symfony 2.4 and Symfony 2.5, ``null`` values won't be validated by
+    the Expression constraint. Therefore, to disallow ``null`` values, you
+    would also have to use the :doc:`NotNull constraint </reference/constraints/NotNull>`.
+
 The :ref:`expression <reference-constraint-expression-option>` option is the
 expression that must return true in order for validation to pass. To learn
 more about the expression language syntax, see