Add note on multirobot refactor to migration guide

migration/Jazzy.rst

@@ -99,3 +99,16 @@ In `PR #4675 <https://github.com/ros-navigation/navigation2/pull/4675>`_ a ``pub
 Default value:
 
 - true
+
+Revamped multirobot bringup and config files to use namespaces
+*******************************************************
+
+In `PR #4715 <https://github.com/ros-navigation/navigation2/pull/4715>`_ multirobot bringup and the use of namespaces were overhauled to be compatible out of the box with ROS namespaces and remove custom logic, specifically:
+
+* The ``use_namespace`` parameter has been removed from ``nav2_bringup`` launch files. The ``namespace`` parameter will now always be used and default to ``/`` for "global namespace".
+* There is now a single rviz config file for both normal and namespaced robots. Topics have been changed to a relative path (i.e. ``/map`` -> ``map``) and the rviz ``namespace`` will be added automatically.
+* There is now a single ``nav2_params.yaml`` config file for both single and multirobot bringup. All the topics have been changed to relative (i.e. ``/scan`` -> ``scan``).
+
+Note that some plugins / nodes might have their own local namespace. This is the case for ``CostmapLayer`` which will be in a ``/ns/[layer_name]`` namespace. For these, a new function ``joinWithParentNamespace`` has been added to make sure joining relative paths results in ``/ns/topic_name`` rather than ``/ns/[layer_name]/topic_name``.
+
+If your use case doesn't require multiple robots, keeping absolute paths in your ``nav2_params.yaml`` config file and rviz config file will preserve existing behavior.