Update Iron.rst (#455)

* Update Iron.rst

add new type of multirobot simulation launch

* Update migration/Iron.rst

---------

Co-authored-by: Steve Macenski <stevenmacenski@gmail.com>

migration/Iron.rst

@@ -5,6 +5,13 @@ Iron to Jazzy
 
 Moving from ROS 2 Iron to Jazzy, a number of stability improvements were added that we will not specifically address here.
 
+Introduce a new Multi-Robot Bringup Launch
+******************************************
+
+`PR #3572 <https://github.com/ros-planning/navigation2/pull/3572>`_ introduces a new way of bringup tb3 multi-robot that names as ``cloned_tb3_simulation_launch.py`` for simulation. ``cloned_tb3_simulation_launch.py`` enables to bring up multiple robots with same parameter that described in ``nav2_multirobot_param_all.yaml``. And multiple robots are separeted by namespaces which are given as a Launch Arguments.
+Existing ``multi_tb3_simulation_launch.py`` which was utilized in previous is replaced with ``unique_tb3_simulation_launch.py``, allowing for multiple unique robot instances utilizing ``nav2_multirobot_params_<N>.yaml`` configuration files.
+
+
 New option for the Voxel and Obstacle Layers
 ********************************************
 `PR #3612 <https://github.com/ros-planning/navigation2/pull/3612>`_ adds a new MaxWithoutUnknownOverwrite option to combination_method parameter in Voxel and Obstacle Layers. This can be used to make sure that the static map is the dominant source of information, and