Add note about multi-robot launch file changes

[TannerGilbert]

Documentation changes for ros-navigation/navigation2#4811.

Should I add an example of what was the problem with the original parsing namely that something like the following didn't work as expected:

IncludeLaunchDescription(
    PythonLaunchDescriptionSource(
        os.path.join(get_package_share_directory('nav2_bringup'), "launch", "cloned_multi_tb3_simulation_launch.py")
    ),
    launch_arguments={"robots": "robot1={x: 0.5, y: 0.5, yaw: 1.5707}"}.items(),
)

or is this not relevant for the documentation?

[SteveMacenski]

Just waiting on CI so I can check the output