pd control 文档中未对 service-gc-safepoint 进行描述

[zhaoxinyu]

Error 报告

This repository is ONLY used to solve problems related to DOCS-CN.
For other issues (TiDB, TiKV, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. What is the document directory for the problem?

https://docs.pingcap.com/zh/tidb/stable/pd-control

2. How would you like to improve it?

加上对 service-gc-safepoint 的相关描述

[zhaoxinyu]

/assign @nolouch