update tikv-control from pingcap#1134 pr

Via: pingcap/docs#1134

Author: yikeke

tools/tikv-control.md

@@ -203,8 +203,12 @@ $ tikv-ctl --host 127.0.0.1:20160 region-properties -r 2
 - `-v` 用于指定配置值。
 
 ```bash
+# 设置 `shared block cache` 的大小.
 $ tikv-ctl modify-tikv-config -m storage -n block_cache.capacity -v 10GB
 success!
+# 当禁用 `shared block cache` 时，为 `write` CF 设置 `block cache size`
+$ tikv-ctl modify-tikv-config -m kvdb -n write.block_cache_size -v 256MB
+success!
 $ tikv-ctl modify-tikv-config -m kvdb -n max_background_jobs -v 8
 success!
 $ tikv-ctl modify-tikv-config -m raftdb -n default.disable_auto_compactions -v true