br: update restriction of ratelimit and concurrency (#15239) (#17840)

Author: ti-chi-bot

br/br-auto-tune.md

@@ -13,7 +13,7 @@ To reduce the impact of backup tasks on the cluster, starting from TiDB v5.4.0,
 
 If you want to reduce the impact of backup tasks on the cluster, you can enable the auto-tune feature. With this feature enabled, BR performs backup tasks as fast as possible without excessively affecting the cluster.
 
-Alternatively, you can limit the backup speed by using the TiKV configuration item [`backup.num-threads`](/tikv-configuration-file.md#num-threads-1) or using the parameter `--ratelimit`.
+Alternatively, you can limit the backup speed by using the TiKV configuration item [`backup.num-threads`](/tikv-configuration-file.md#num-threads-1) or using the parameter `--ratelimit`. When `--ratelimit` is set, to avoid too many tasks causing the speed limit to fail, the `concurrency` parameter of br is automatically adjusted to `1`.
 
 ## Use auto-tune