Document until parameter

Signed-off-by: Shane Loretz <sloretz@openrobotics.org>

rclpy/src/rclpy/event.hpp

@@ -40,6 +40,7 @@ class Event
 public:
  /// Wait until a time specified by a system or steady clock.
  /// \param clock the clock to use for time synchronization with until
+ /// \param until this method will block until this time is reached.
  template<typename ClockType>
  void wait_until(std::shared_ptr<Clock> clock, rcl_time_point_t until);
 
@@ -48,6 +49,7 @@ class Event
  /// the target ROS time is reached.
  /// when a given ROS time is reached.
  /// \param clock the clock to use for time synchronization.
+ /// \param until this method will block until this time is reached.
  void wait_until_ros(std::shared_ptr<Clock> clock, rcl_time_point_t until);
 
  /// Indicate if the Event is set.