Add a comment about __instancecheck__

Author: tomasr8

Python/optimizer_bytecodes.c

@@ -987,6 +987,8 @@ dummy_func(void) {
                     }
                     PyTypeObject *cls_o = (PyTypeObject *)sym_get_const(ctx, item);
                     if (cls_o &&
+                        // Ensure that item is an exact instance of `type` ensuring that
+                        // there is no __instancecheck__ defined.
                         sym_matches_type(item, &PyType_Type) &&
                         (inst_type == cls_o || PyType_IsSubtype(inst_type, cls_o)))
                     {