py: Clarify API for map/set lookup when removing&adding at once.

Addresses issue #1160.

py/objset.c

@@ -427,7 +427,7 @@ STATIC mp_obj_t set_symmetric_difference_update(mp_obj_t self_in, mp_obj_t other
     mp_obj_t iter = mp_getiter(other_in);
     mp_obj_t next;
     while ((next = mp_iternext(iter)) != MP_OBJ_STOP_ITERATION) {
-        mp_set_lookup(&self->set, next, MP_MAP_LOOKUP_REMOVE_IF_FOUND | MP_MAP_LOOKUP_ADD_IF_NOT_FOUND);
+        mp_set_lookup(&self->set, next, MP_MAP_LOOKUP_ADD_IF_NOT_FOUND_OR_REMOVE_IF_FOUND);
     }
     return mp_const_none;
 }