tikv / titan: Add note for disabling titan (#20996)

Author: qiancai

storage-engine/titan-configuration.md

@@ -138,7 +138,7 @@ To disable Titan, you can configure the `rocksdb.defaultcf.titan.blob-run-mode`
 - When the option is set to `read-only`, all newly written values are written into RocksDB, regardless of the value size.
 - When the option is set to `fallback`, all newly written values are written into RocksDB, regardless of the value size. Also, all compacted values stored in the Titan blob file are automatically moved back to RocksDB.
 
-To fully disable Titan for all existing and future data, you can follow these steps. Note that you can skip Step 2 because it greatly impacts online traffic performance. In fact even without Step 2, the data compaction consumes extra I/O and CPU resources when it moves data from Titan to RocksDB, and performance will degrade (sometimes as much as 50%) when TiKV I/O or CPU resources are limited.
+To disable Titan for all existing and future data, you can follow these steps. Note that you can skip Step 2 because it greatly impacts online traffic performance. In fact even without Step 2, the data compaction consumes extra I/O and CPU resources when it moves data from Titan to RocksDB, and performance will degrade (sometimes as much as 50%) when TiKV I/O or CPU resources are limited.
 
 1. Update the configuration of the TiKV nodes you wish to disable Titan for. You can update configuration in two methods:
 
@@ -167,7 +167,11 @@ To fully disable Titan for all existing and future data, you can follow these st
 
 3. After the compaction is finished, wait for the **Blob file count** metrics under **TiKV-Details**/**Titan - kv** to decrease to `0`.
 
-4. Update the configuration of these TiKV nodes to disable Titan.
+4. For TiDB v8.5.0 or a later version, update the configuration of these TiKV nodes to disable Titan.
+
+    > **Warning:**
+    >
+    > For versions earlier than v8.5.0, it is recommended to skip this step, because it might cause TiKV to crash. In these earlier versions, following Step 1 is sufficient to disable Titan. There is no performance difference between the configuration changes in Step 1 and the following changes in this step after the data migration is complete.
 
     ```toml
     [rocksdb.titan]