clarify constraints on non-submitted forms

Author: xabbuh

form/without_class.rst

@@ -112,3 +112,9 @@ but here's a short example::
     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>`.
+
+.. caution::
+
+    If a form is not submitted with the request (for example, when a form
+    is only partially submitted in a HTTP PATCH request), its validation
+    constraints will be ignored.