tiup: add a ref to limitation on users operation during upgrade using…

… tiup (#17952) (#18141)

upgrade-tidb-using-tiup.md

@@ -52,6 +52,8 @@ 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.