*: add a limit for smooth upgrade (pingcap#13713)

smooth-upgrade-tidb.md

@@ -39,6 +39,8 @@ When using the smooth upgrade feature, note the following limitations.
 
 * Before the upgrade, if there is a canceling DDL job in the cluster, that is, an ongoing DDL job is being canceled by a user, because the job in the canceling state cannot be paused, TiDB will retry canceling the job. If the retry fails, an error is reported and the upgrade is exited.
 
+* In scenarios of using TiUP to upgrade TiDB, because TiUP upgrade has a timeout period, if the cluster has a large number of DDL jobs (more than 300) waiting in queues before the upgrade, the upgrade might fail.
+
 * During the upgrade, the following operations are not allowed:
 
     * Run DDL operations on system tables (`mysql.*`, `information_schema.*`, `performance_schema.*`, and `metrics_schema.*`).