add explanation when timeout is equal to 0.

Signed-off-by: Tomoya Fujita <Tomoya.Fujita@sony.com>

rclcpp/include/rclcpp/executor.hpp

@@ -306,6 +306,7 @@ class Executor
    * become ready, this method will execute work repeatedly until `max_duration` has elapsed.
    *
    * \param[in] max_duration The maximum amount of time to spend executing work, must be >= 0.
+   *   `0` means it keeps executing until no more work is available.
    * \throw throw std::invalid_argument if max_duration is less than 0.
    * Note that spin_all() may take longer than this time as it only returns once max_duration has
    * been exceeded.

rclcpp/include/rclcpp/executors/static_single_threaded_executor.hpp

@@ -101,6 +101,7 @@ class StaticSingleThreadedExecutor : public rclcpp::Executor
   /**
    * This non-blocking function will execute entities until timeout (must be >= 0)
    * or no more work available.
+   * If timeout is `0`, this keeps executing until no more work is available.
    * If new entities get ready while executing work available, they will be executed
    * as long as the timeout hasn't expired.
    *