Update Expression constraint with "negate" option

Author: Florent Mata

reference/constraints/Expression.rst

@@ -143,6 +143,11 @@ expression that must return true in order for validation to pass. To learn
 more about the expression language syntax, see
 :doc:`/components/expression_language/syntax`.
 
+.. versionadded:: 6.2
+
+    Alternatively, you can set the `negate`_ option to ``false`` in order to
+    assert that the expression must return true for validation to fail.
+
 .. sidebar:: Mapping the Error to a Specific Field
 
     You can also attach the constraint to a specific property and still validate
@@ -294,6 +299,13 @@ Parameter        Description
 ``{{ label }}``  Corresponding form field label
 ===============  ==============================================================
 
+``negate``
+~~~~~~~~~~~
+
+**type**: ``boolean`` **default**: ``true``
+
+If ``false``, the validation fails when expression returns ``true``.
+
 .. include:: /reference/constraints/_payload-option.rst.inc
 
 ``values``