pd: add add description of pd status

[Lloyd-Pottiger]

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.

There are no description of pd status. like L and UI.

2. Describe your suggestion or addition.

Please add it into dashboard/dashboard-cluster-info.md

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

tiup cluster display <cluster-name>

[Oreoxmt]

@AndreMouche PTAL