tikv: clarify reserve-space zero values (#22813) (#22816)

Author: ti-chi-bot

tikv-configuration-file.md

@@ -460,9 +460,9 @@ Configuration items related to storage.
 
 + When TiKV is started, some space is reserved on the disk as disk protection. When the remaining disk space is less than the reserved space, TiKV restricts some write operations. The reserved space is divided into two parts: 80% of the reserved space is used as the extra disk space required for operations when the disk space is insufficient, and the other 20% is used to store the temporary file. In the process of reclaiming space, if the storage is exhausted by using too much extra disk space, this temporary file serves as the last protection for restoring services.
 + The name of the temporary file is `space_placeholder_file`, located in the `storage.data-dir` directory. When TiKV goes offline because its disk space ran out, if you restart TiKV, the temporary file is automatically deleted and TiKV tries to reclaim the space.
-+ When the remaining space is insufficient, TiKV does not create the temporary file. The effectiveness of the protection is related to the size of the reserved space. The size of the reserved space is the larger value between 5% of the disk capacity and this configuration value. When the value of this configuration item is `"0MiB"`, TiKV disables this disk protection feature.
++ When the remaining space is insufficient, TiKV does not create the temporary file. The effectiveness of the protection is related to the size of the reserved space. The size of the reserved space is the larger value between 5% of the disk capacity and this configuration value. If this configuration item is set to `0`, or to a zero value with any supported unit (for example, `0KiB`, `0MiB`, or `0GiB`), TiKV disables this disk protection feature.
 + Default value: `"5GiB"`
-+ Unit: MiB|GiB
++ Unit: B|KB|KiB|MB|MiB|GB|GiB|TB|TiB|PB|PiB
 
 ### `enable-ttl`