api/event: Document a missing stream-event op.

This is the inverse of `occupy`.  We don't actually *do* anything with
`occupy`, so we don't with this either.

Author: gnprice

src/api/eventTypes.js

@@ -121,6 +121,7 @@ export type StreamEvent =
   | {| ...StreamListEvent, op: 'create', |}
   | {| ...StreamListEvent, op: 'delete', |}
   | {| ...StreamListEvent, op: 'occupy', |}
+  | {| ...StreamListEvent, op: 'vacate', |}
   | {|
       ...EventCommon,
       type: typeof EventTypes.stream,

src/streams/streamsReducers.js

@@ -43,6 +43,7 @@ export default (state: StreamsState = initialState, action: Action): StreamsStat
               );
 
             case 'occupy':
+            case 'vacate':
               return state;
 
             default:

src/subscriptions/subscriptionsReducers.js

@@ -71,6 +71,7 @@ export default (state: SubscriptionsState = initialState, action: Action): Subsc
             case 'create':
             case 'delete':
             case 'occupy':
+            case 'vacate':
               return state;
 
             default: