Add some words

docs/tcp_replication.md

@@ -202,6 +202,8 @@ Asks the server for the current position of all streams.
 
    The server should clear all associated user sync data from the worker.
 
+   This is used when a worker is shutting down.
+
 #### FEDERATION_ACK (C)
 
    Acknowledge receipt of some federation data

synapse/server.py

@@ -243,6 +243,9 @@ class HomeServer(object):
 
     def get_instance_id(self):
         """A unique ID for this synapse process instance.
+
+        This is used to distinguish running instances in worker-based
+        deployments.
         """
         return self.instance_id