py/modbuiltins: __repl_print__: Add comment about setting "_" special var.

py/modbuiltins.c

@@ -439,6 +439,7 @@ STATIC mp_obj_t mp_builtin___repl_print__(mp_obj_t o) {
         mp_print_str(&mp_plat_print, "\n");
         #endif
         #if MICROPY_CAN_OVERRIDE_BUILTINS
+        // Set "_" special variable
         mp_obj_t dest[2] = {MP_OBJ_SENTINEL, o};
         mp_type_module.attr(MP_OBJ_FROM_PTR(&mp_module_builtins), MP_QSTR__, dest);
         #endif