[action_client] Defining that action diagram is with respect to the client.

Author: Murilo Marinho

docs/source/action_clients.rst

@@ -15,6 +15,11 @@ that when deploying actions in real applications they will be part of a more com
 publishers, subscribers, service servers/clients, and other actions server/clients. Therefore it is important to take
 this complexity in consideration when designing your packages to make sure that an :code:`Action` is the best way to communicate.
 
+Diagram
+-------
+
+This is the sequence diagram from the point of view of the action client.
+
 .. mermaid::
 
     ---
@@ -34,6 +39,9 @@ this complexity in consideration when designing your packages to make sure that
       end
       Action Server -->>- Action Client: ActionClientNode.action_result_callback()
 
+Overview
+--------
+
 #. Create the Node with an :code:`ActionClient`.
 #. Update the :file:`setup.py` so that :program:`ros2 run` finds the node (if needed).