PD 微服务使用文档，能否增加关于切换primary的描述

[shawn0915]

Change Request

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. Describe what you find is inappropriate or missing in the existing docs.

2. Describe your suggestion or addition.

详见：
https://asktug.com/t/topic/1028789

3. Provide some reference materials (documents, websites, etc) if you could.

[rleungx]

tikv/pd#8157 is working on it.