Improve upgrade limitations (pingcap#18332)

smooth-upgrade-tidb.md

@@ -73,12 +73,16 @@ You can take the following steps to upgrade TiDB manually or by using a script:
 
 When using the smooth upgrade feature, note the following limitations.
 
+> **Note:**
+>
+> The limitations in this section apply not only to scenarios using the smooth upgrade feature, but also to [upgrading TiDB using TiUP](/upgrade-tidb-using-tiup.md#upgrade-tidb-using-tiup).
+
 ### Limitations on user operations
 
 * Before the upgrade, consider the following restrictions:
 
  * 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.
- * If your current TiDB version is earlier than v8.1.0 and the TiDB Distributed eXecution Framework (DXF) is enabled, disable it by setting [`tidb_enable_dist_task`](/system-variables.md#tidb_enable_dist_task-new-in-v710) to `OFF`. Make sure that all ongoing distributed `ADD INDEX` and `IMPORT INTO` tasks are completed. Alternatively, you can cancel these tasks and wait until the upgrade is complete to restart them. Otherwise, the `ADD INDEX` operations during the upgrade might cause data index inconsistency.
+ * If your current TiDB version is earlier than v8.1.0 and the TiDB Distributed eXecution Framework (DXF) is enabled, disable it by setting [`tidb_enable_dist_task`](/system-variables.md#tidb_enable_dist_task-new-in-v710) to `OFF`. Make sure that all ongoing distributed `ADD INDEX` and `IMPORT INTO` tasks are completed. Alternatively, you can cancel these tasks and wait until the upgrade is complete to restart them. Otherwise, the `ADD INDEX` operations during the upgrade might cause data index inconsistency. If your current TiDB version is v8.1.0 or later, you do not need to disable the DXF, and you can just ignore this limitation.
 
 * 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.
 

upgrade-tidb-using-tiup.md

@@ -19,8 +19,8 @@ This document is targeted for the following upgrade paths:
 > 1. You cannot upgrade TiFlash online from versions earlier than 5.3 to 5.3 or later. Instead, you must first stop all the TiFlash instances of the early version, and then upgrade the cluster offline. If other components (such as TiDB and TiKV) do not support an online upgrade, follow the instructions in warnings in [Online upgrade](#online-upgrade).
 > 2. **DO NOT** run DDL statements during the upgrade process. Otherwise, the issue of undefined behavior might occur.
 > 3. **DO NOT** upgrade a TiDB cluster when a DDL statement is being executed in the cluster (usually for the time-consuming DDL statements such as `ADD INDEX` and the column type changes). Before the upgrade, it is recommended to use the [`ADMIN SHOW DDL`](/sql-statements/sql-statement-admin-show-ddl.md) command to check whether the TiDB cluster has an ongoing DDL job. If the cluster has a DDL job, to upgrade the cluster, wait until the DDL execution is finished or use the [`ADMIN CANCEL DDL`](/sql-statements/sql-statement-admin-cancel-ddl.md) command to cancel the DDL job before you upgrade the cluster.
->
-> If the TiDB version before upgrade is v7.1.0 or later, you can ignore the preceding warnings 2 and 3. For more information, see [TiDB Smooth Upgrade](/smooth-upgrade-tidb.md).
+> 4. If the TiDB version before upgrade is 7.1.0 or later, you can ignore the preceding warnings 2 and 3. For more information, see [limitations on using TiDB smooth upgrade](/smooth-upgrade-tidb.md#limitations).
+> 5. Be sure to read [limitations on user operations](/smooth-upgrade-tidb.md#limitations-on-user-operations) before upgrading your TiDB cluster using TiUP.
 
 > **Note:**
 >
@@ -53,8 +53,6 @@ This document is targeted for the following upgrade paths:
 >   If the original cluster is v7.1.0 or earlier, when upgrading to v7.2.0 or later, because of the introduction of [`performance.lite-init-stats`](/tidb-configuration-file.md#lite-init-stats-new-in-v710), the statistics loading time is greatly reduced. In this case, the `init stats info time` before the upgrade is longer than the loading time after the upgrade.
 >  - If you want to shorten the rolling upgrade duration of TiDB and the potential performance impact of missing initial statistical information during the upgrade is acceptable for your cluster, you can set `performance.force-init-stats` to `OFF` before the upgrade by [modifying the configuration of the target instance with TiUP](/maintain-tidb-using-tiup.md#modify-the-configuration). After the upgrade is completed, you can reassess and revert this setting if necessary.
 
-There are some other [limitations on user operations](/smooth-upgrade-tidb.md#limitations-on-user-operations) when you upgrade TiDB using TiUP. It is strongly recommended that you read those limitations before upgrade.
-
 ## Upgrade caveat
 
 - TiDB currently does not support version downgrade or rolling back to an earlier version after the upgrade.