document the REPLICATE command a bit better (#6305)

since I found myself wonder how it works

synapse/replication/tcp/client.py

@@ -16,10 +16,17 @@
 """
 
 import logging
+from typing import Dict
 
 from twisted.internet import defer
 from twisted.internet.protocol import ReconnectingClientFactory
 
+from synapse.replication.slave.storage._base import BaseSlavedStore
+from synapse.replication.tcp.protocol import (
+    AbstractReplicationClientHandler,
+    ClientReplicationStreamProtocol,
+)
+
 from .commands import (
     FederationAckCommand,
     InvalidateCacheCommand,
@@ -27,7 +34,6 @@ from .commands import (
     UserIpCommand,
     UserSyncCommand,
 )
-from .protocol import ClientReplicationStreamProtocol
 
 logger = logging.getLogger(__name__)
 
@@ -42,7 +48,7 @@ class ReplicationClientFactory(ReconnectingClientFactory):
 
     maxDelay = 30  # Try at least once every N seconds
 
-    def __init__(self, hs, client_name, handler):
+    def __init__(self, hs, client_name, handler: AbstractReplicationClientHandler):
         self.client_name = client_name
         self.handler = handler
         self.server_name = hs.config.server_name
@@ -68,13 +74,13 @@ class ReplicationClientFactory(ReconnectingClientFactory):
         ReconnectingClientFactory.clientConnectionFailed(self, connector, reason)
 
 
-class ReplicationClientHandler(object):
+class ReplicationClientHandler(AbstractReplicationClientHandler):
     """A base handler that can be passed to the ReplicationClientFactory.
 
     By default proxies incoming replication data to the SlaveStore.
     """
 
-    def __init__(self, store):
+    def __init__(self, store: BaseSlavedStore):
         self.store = store
 
         # The current connection. None if we are currently (re)connecting
@@ -138,11 +144,13 @@ class ReplicationClientHandler(object):
         if d:
             d.callback(data)
 
-    def get_streams_to_replicate(self):
+    def get_streams_to_replicate(self) -> Dict[str, int]:
         """Called when a new connection has been established and we need to
         subscribe to streams.
 
-        Returns a dictionary of stream name to token.
+        Returns:
+            map from stream name to the most recent update we have for
+            that stream (ie, the point we want to start replicating from)
         """
         args = self.store.stream_positions()
         user_account_data = args.pop("user_account_data", None)