cloud: update tidb_dml_type limitation (#22415) (#22418)

Author: ti-chi-bot

system-variables.md

@@ -1954,6 +1954,10 @@ Assume that you have a cluster with 4 TiDB nodes and multiple TiKV nodes. In thi
 >
 > The bulk DML execution mode (`tidb_dml_type = "bulk"`) is an experimental feature. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an [issue](https://github.com/pingcap/tidb/issues). In the current version, when TiDB performs large transactions using the bulk DML mode, it might affect the memory usage and execution efficiency of TiCDC, TiFlash, and the resolved-ts module of TiKV, and might cause OOM issues. Additionally, BR might be blocked and fail to process when encountering locks. Therefore, it is not recommended to use this mode when these components or features are enabled.
 
+> **Note:**
+>
+> This variable is read-only for [{{{ .starter }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#starter) and [{{{ .essential }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#essential).
+
 - Scope: SESSION
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
 - Type: String