Merge branch '6.4' into 7.3

* 6.4:
  [Form] Mention conditional constraitns via form validation groups

forms.rst

@@ -1621,6 +1621,12 @@ To achieve this, use the ``expression`` option of the
         ])
     ;
 
+.. seealso::
+
+    Another way to implement conditional constraints is to configure the
+    ``validation_groups`` form option with a callable. See
+    :doc:`/form/validation_groups`.
+
 Troubleshooting
 ---------------