refine code, add comments for stat on_disconnect.

@@ -206,6 +206,7 @@ int SrsRtmpConn::do_cycle()
     }
     
     ret = service_cycle();
+    
     http_hooks_on_close();
     SrsStatistic* stat = SrsStatistic::instance();
     stat->on_disconnect(_srs_context->get_id());

@@ -161,6 +161,9 @@ public:
     virtual int on_client(int id, SrsRequest* req);
     /**
      * client disconnect
+     * @remark the on_disconnect always call, while the on_client is call when
+     *      only got the request object, so the client specified by id maybe not
+     *      exists in stat.
      */
     virtual void on_disconnect(int id);
     /**