doc: document neighbor and mount idle subsystems

doc/protocol.rst

@@ -413,6 +413,8 @@ Querying :program:`MPD`'s status
     - ``sticker``: the sticker database has been modified.
     - ``subscription``: a client has subscribed or unsubscribed to a channel
     - ``message``: a message was received on a channel this client is subscribed to; this event is only emitted when the queue is empty
+    - ``neighbor``: a neighbor was found or lost
+    - ``mount``: the mount list has changed
 
     Change events accumulate, even while the connection is not in
     "idle" mode; no events gets lost while the client is doing