update dttool description

Signed-off-by: hehechen <[email protected]>

tiflash/tiflash-command-line-flags.md

@@ -29,8 +29,8 @@ This document introduces the command-line flags that you can use when you launch
     - `--version`: The version of DTFile. The value options are `1` and `2` (default). `1` is the old version, and `2` is the version corresponding to the new checksum.
     - `--algorithm`: The hash algorithm used for data validation. The value options are `xxh3` (default), `city128`, `crc32`, `crc64`, and `none`. This parameter is effective only when `version` is `2`.
     - `--frame`: The size of the validation frame. The default value is `1048576`. This parameter is effective only when `version` is `2`.
-    - `--compression`: The target compression algorithm. The value options are `lz4` (default), `lz4hc`, `zstd`, and `none`.
-    - `--level`: The target compression level. The default value is `-1`, which means automatic mode. The value range varies with different compression algorithms.
+    - `--compression`: The target compression algorithm. The value options are `LZ4` (default), `LZ4HC`, `zstd`, and `none`.
+    - `--level`: The target compression level. If not specified, the recommended compression level is automatically used by default according to the compression algorithm. If compression is set to LZ4, the default level is 1; if compression is set to zstd, the default level is 1; if compression is set to LZ4HC, the default level is 9. The value range varies with different compression algorithms.
     - `--config-file`: The configuration file of `dttool migrate` is the same as the [configuration file of `server`](/tiflash/tiflash-command-line-flags.md#server---config-file). When you use the configuration file, exit the local TiFlash server instance. For more information, see `--imitative`.
     - `--file-id`: The ID of the DTFile. For example, the ID of the DTFile `dmf_123` is `123`.
     - `--workdir`: The parent directory of `dmf_xxx`.