server: Document `diconnect`'s idempotency requirement

Suggested-by: @azuz

jvm/src/main/scala/cs214/webapp/server/web/ServerApp.scala

@@ -103,6 +103,7 @@ private[web] abstract class ServerApp:
           ()
       }
 
+  /** Records a disconnection.  May be called multiple times (should be idempotent). **/
   def disconnect(userId: UserId, channel: WebSocketChannel): Unit = instanceLock.synchronized:
     if channels(userId).remove(channel) then
       println(f"[${appInfo.id}/$instanceId/$userId] client disconnected")