feat(queue): document protocol between agent and leader

Document leader's states

apps/emqx_ds_shared_sub/README.md

@@ -7,7 +7,17 @@ This application makes durable session capable to cooperatively replay messages
 ![General layout](docs/images/ds_shared_subs.png)
 
 * The nesting reflects nesting/ownership of entity states.
-* The bold arrow represent the [most complex interaction](https://github.com/emqx/eip/blob/main/active/0028-durable-shared-subscriptions.md#shared-subscription-session-handler), between session-side group subscription state machine and the shared subscription leader.
+* The bold arrow represent the [most complex interaction](https://github.com/emqx/eip/blob/main/active/0028-durable-shared-subscriptions.md#shared-subscription-session-handler), between session-side group subscription state machine (**GroupSM**) and the shared subscription leader (**Leader**).
+
+# GroupSM and Leader communication
+
+The target state of GroupSM and its representation in Leader is `replaying`. That is, when the GroupSM and the Leader agree on the leased streams, Leader sends lease confirmations to the GroupSM, the GroupSM sends iteration updates.
+
+Other states are used to gracefully reassign streams to the GroupSM.
+
+Below is the sequence diagram of the interaction.
+
+![GroupSM and Leader communication](docs/images/groupsm_leader_communication.png)
 
 # Contributing