Update tidb_dml_batch_size section (#7371)

system-variables.md

@@ -487,6 +487,13 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
     In the case of a poor network environment, appropriately increasing the value of this variable can effectively alleviate error reporting to the application end caused by timeout. If the application end wants to receive the error information more quickly, minimize the value of this variable.
 
+### tidb_batch_insert
+
+- Scope: SESSION
+- Default value: `OFF`
+- This variable permits `tidb_dml_batch_size` to be used in `INSERT` statements.
+- Only the value `OFF` provides ACID compliance. Setting this to any other value breaks the atomicity and isolation guarantees of TiDB, because an individual `INSERT` statement will be split into smaller transactions.
+
 ### tidb_broadcast_join_threshold_count <span class="version-mark">New in v5.0</span>
 
 - Scope: SESSION | GLOBAL
@@ -638,6 +645,7 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - Range: `[0, 2147483647]`
 - Unit: Rows
 - When this value is greater than `0`, TiDB will batch commit statements such as `INSERT` or `LOAD DATA` into smaller transactions. This reduces memory usage and helps ensure that the `txn-total-size-limit` is not reached by bulk modifications.
+- To use `tidb_dml_batch_size` in `INSERT` statements, you also need to set the system variable `tidb_batch_insert` to `ON`.
 - Only the value `0` provides ACID compliance. Setting this to any other value will break the atomicity and isolation guarantees of TiDB.
 
 ### tidb_enable_1pc <span class="version-mark">New in v5.0</span>