Change docs to express what is possible instead of what is not

Co-authored-by: Tomoya Fujita <Tomoya.Fujita@sony.com>
Signed-off-by: Matthias Holoch <mholoch@gmail.com>

rclcpp/include/rclcpp/executor.hpp

@@ -347,7 +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.
+   * The implementation is suitable for a single-thread model of execution.
    * \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.