document the REPLICATE command a bit better (#6305)

since I found myself wonder how it works

synapse/replication/slave/storage/_base.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import logging
+from typing import Dict
 
 import six
 
@@ -44,7 +45,14 @@ class BaseSlavedStore(SQLBaseStore):
 
         self.hs = hs
 
-    def stream_positions(self):
+    def stream_positions(self) -> Dict[str, int]:
+        """
+        Get the current positions of all the streams this store wants to subscribe to
+
+        Returns:
+            map from stream name to the most recent update we have for
+            that stream (ie, the point we want to start replicating from)
+        """
         pos = {}
         if self._cache_id_gen:
             pos["caches"] = self._cache_id_gen.get_current_token()