Improve description for notification

Author: emhane

discv5/discv5-wire.md

@@ -156,7 +156,9 @@ handshake packet.
 
 ### Notification Packet (`flag = 3`)
 
-Aside from the flag value, this container is identical to a [message packet](#ordinary-message-packet).
+Aside from the flag value, this container is identical to a
+[message packet](#ordinary-message-packet). Contrary to a message packet, failing to
+decrypt a notification packet should not trigger a handshake.
 
 ## Protocol Messages