Tweak ros2_overview tutorial.

harmonic/ros2_overview.md

@@ -57,12 +57,17 @@ ROS nodes will be intraprocess.
 
 This figure illustrates the concept of composition. The left diagram captures
 the idea of not using composition. All the three example nodes are standalone
-nodes, and they can talk via interprocess communication. The center diagram
-represents the scenario where we use composition and we start our own container
-from our own launch file. All communication is intraprocess here. The right
-diagram is still using composition but the launch file doesn't start the
-container directly. This setup by itself will not work until you start an
-external ROS container (manually or via a separate launch file).
+nodes, and they can talk via interprocess communication using the bridge.
+The center diagram represents the scenario where we can use composition and our
+own ROS container between Gazebo and the bridge, but there's an additional
+consumer node outside that we cannot control. All communication between Gazebo
+and the bridge is intraprocess and interprocess between the external consumer
+node and the bridge.
+The diagram on the right side is using composition across all nodes but the
+launch file doesn't start our own container directly. This setup by itself will
+not work until you start an external ROS container (manually or via a separate launch file). In this diagram, the external ROS consumer node starts the
+container. We're using the Nav2 logo as an example of external ROS 2 consumer
+node.
 
 You can learn more about ROS composition in [this tutorial](https://docs.ros.org/en/galactic/Tutorials/Intermediate/Composition.html).