Add basic docstring to annotate_context_with_state

synapse/state.py

@@ -137,6 +137,17 @@ class StateHandler(object):
 
     @defer.inlineCallbacks
     def annotate_context_with_state(self, event, context, old_state=None):
+        """ Fills out the context with the `current state` of the graph. The
+        `current state` here is defined to be exclusive of the given event,
+        i.e. its the state of the event graph just before the event.
+
+        If `event` has `auth_events` then this will also fill out the
+        `auth_events` field on `context` from the `current_state`.
+
+        Args:
+            event (EventBase)
+            context (EventContext)
+        """
         yield run_on_reactor()
 
         if old_state: