Update get_current_hosts_in_room docstring

Update the docstring for `get_current_hosts_in_room` to mention the
blocking behaviour for rooms with partial state. Also document the order
of returned hosts.

Signed-off-by: Sean Quah <[email protected]>

synapse/storage/controllers/state.py

@@ -525,7 +525,15 @@ async def get_current_state_event(
         return state_map.get(key)
 
     async def get_current_hosts_in_room(self, room_id: str) -> List[str]:
-        """Get current hosts in room based on current state."""
+        """Get current hosts in room based on current state.
+
+        Blocks until we have full state for the given room. This only happens for rooms
+        with partial state.
+
+        Returns:
+            A list of hosts in the room, sorted by longest in the room first. (aka.
+            sorted by join with the lowest depth first).
+        """
 
         await self._partial_state_room_tracker.await_full_state(room_id)