tidb: add description for var tidb_ddl_reorg_max_write_speed (#19445)

Author: CbcWestwolf

system-variables.md

@@ -1695,6 +1695,17 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - This variable is used to set the priority of executing the `ADD INDEX` operation in the `re-organize` phase.
 - You can set the value of this variable to `PRIORITY_LOW`, `PRIORITY_NORMAL` or `PRIORITY_HIGH`.
 
+### tidb_ddl_reorg_max_write_speed <span class="version-mark">New in v8.5.0</span>
+
+- Scope: GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
+- Type: Integer
+- Default value: `0`
+- Range: `[0, 1125899906842624]` (the maximum value that can be set is 1 PiB)
+- This variable limits the write bandwidth for each TiKV node and only takes effect when index creation acceleration is enabled (controlled by the [`tidb_ddl_enable_fast_reorg`](#tidb_ddl_enable_fast_reorg-new-in-v630) variable). When the data size in your cluster is quite large (such as billions of rows), limiting the write bandwidth for index creation can effectively reduce the impact on application workloads.
+- The default value `0` means no write bandwidth limit. The default unit is bytes per second. You can also set the value in formats such as `'1GiB'` or `'256MiB'`.
+
 ### tidb_ddl_reorg_worker_cnt
 
 > **Note:**