Add graceful cancellation API to the migration guides

Signed-off-by: Ramon Wijnands <[email protected]>

migration/Iron.rst

@@ -294,3 +294,7 @@ New BtActionServer/BtNavigator parameter
 
 `PR #4209 <https://github.com/ros-planning/navigation2/pull/4209>`_ introduces a new boolean parameter ``always_reload_bt_xml``, which enables the possibility to always reload a requested behavior tree XML description, regardless of the currently active XML. This allows keeping the action server running while changing/developing the XML description.
 
+New graceful cancellation API for Controllers
+*********************************************
+
+`PR #4136 <https://github.com/ros-planning/navigation2/pull/4136>`_ introduces a new graceful cancellation API for controllers. Previously when a goal was canceled, the controller would stop the robot immediately. This API allows the controller to stop the robot in a more graceful way. The new API is implemented in the ``RegulatedPurePursuitController`` by adding a new parameter ``cancel_deceleration``. So when the goal is canceled, a constant deceleration will be used to stop the robot instead of stopping immediately. This API can be should be added to all controllers that have acceleration limits.