tools: add dynamically modifying RocksDB configuration (#541)

* tools: add dynamically modifying RocksDB configuration

* Update wording

tools/tikv-control.md

@@ -179,3 +179,22 @@ If the command is successfully executed, it prints the above information. If the
     ```bash
     $ tikv-ctl --host 127.0.0.1:20160 region-properties -r 2
     ```
+
+### Modify the RocksDB configuration of TiKV dynamically
+
+You can use the `modify-tikv-config` command to dynamically modify the configuration arguments. Currently, it only supports dynamically modifying RocksDB related arguments. 
+
+- `-m` is used to specify the target RocksDB. You can set it to `kvdb` or `raftdb`.
+- `-n` is used to specify the configuration name. 
+    You can refer to the arguments of `[rocksdb]` and `[raftdb]` (corresponding to `kvdb` and `raftdb`) in the [TiKV configuration template](https://github.com/pingcap/tikv/blob/master/etc/config-template.toml#L213-L500).
+    You can use `default|write|lock + . + argument name` to specify the configuration of different CFs. For `kvdb`, you can set it to `default`, `write`, or `lock`; for `raftdb`, you can only set it to `default`.
+- `-v` is used to specify the configuration value.
+
+```bash
+$ tikv-ctl modify-tikv-config -m kvdb -n max_background_jobs -v 8
+success！
+$ tikv-ctl modify-tikv-config -m kvdb -n write.block-cache-size -v 256MB
+success!
+$ tikv-ctl modify-tikv-config -m raftdb -n default.disable_auto_compactions -v true
+success!
+```