add to docs a note about parallel processing support of spin_until_fu…

…ture_complete

rclcpp/include/rclcpp/executor.hpp

@@ -347,6 +347,7 @@ class Executor
 
   /// Spin (blocking) until the future is complete, it times out waiting, or rclcpp is interrupted.
   /**
+   * Handles one work item after the other, hence, it does not support parallel processing.
    * \param[in] future The future to wait on. If this function returns SUCCESS, the future can be
    *   accessed without blocking (though it may still throw an exception).
    * \param[in] timeout Optional timeout parameter, which gets passed to Executor::spin_node_once.