tiup: restriction about DXF (#19632) (#19636)

upgrade-tidb-using-tiup.md

@@ -172,7 +172,7 @@ To avoid undefined behaviors or other unexpected problems during the upgrade, it
 
 - Cluster DDLs:
 
-    - If you use [smooth upgrade](/smooth-upgrade-tidb.md), you do not need to check the DDL operations of your TiDB cluster. You do not need to wait for the completion of DDL jobs or cancel ongoing DDL jobs.
+    - If you use [smooth upgrade](/smooth-upgrade-tidb.md) to upgrade TiDB to v8.1.0 or later, and the [Distributed eXecution Framework (DXF)](/tidb-distributed-execution-framework.md) is enabled, it is recommended to disable the DXF before upgrading. Otherwise, the indexes added during the upgrade process might be inconsistent with the data, leading to upgrade failures. 
     - If you do not use [smooth upgrade](/smooth-upgrade-tidb.md), it is recommended to use the [`ADMIN SHOW DDL`](/sql-statements/sql-statement-admin-show-ddl.md) statement to check whether ongoing DDL jobs exist. If an ongoing DDL job exists, wait for the completion of its execution or cancel it using the [`ADMIN CANCEL DDL`](/sql-statements/sql-statement-admin-cancel-ddl.md) statement before performing an upgrade.
 
 - Cluster backup: It is recommended to execute the [`SHOW [BACKUPS|RESTORES]`](/sql-statements/sql-statement-show-backups.md) statement to check whether there is an ongoing backup or restore task in the cluster. If yes, wait for its completion before performing an upgrade.