classes.tuple: Add a note about "defining-class"

core/classes/tuple/tuple.factor

@@ -240,6 +240,9 @@ M: tuple-class update-class
         [ define-tuple-prototype ]
     } cleave ;
 
+! The "defining-class" word-prop is to ensure that the tuple being
+! defined becomes a classoid as it's being parsed so that it can
+! be used as the type of its own slots.
 : define-new-tuple-class ( class superclass slots -- )
     {
         [ drop f f tuple-class define-class ]