[#2058] Adding note about where to find the "constraints" form option in the source

Author: weaverryan

reference/forms/types/options/constraints.rst.inc

@@ -5,3 +5,5 @@ constraints
 
 Allows you to attach one or more validation constraints to a specific field.
 For more information, see :ref:`Adding Validation<form-option-constraints>`.
+This option is added in the :class:`Symfony\\Component\\Form\\Extension\\Validator\\Type\\FormTypeValidatorExtension`
+form extension.