Improve related documentation for stale/strong read and relative operation in cloud

[nolouch]

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.

Currently, the document about Stale Read and follower read is not friendly to users:

• [ ] Missing the docs about the session variables. such as closest-replicas
• [ ] Missing the docs on how to use it in the cluster deployed by tidb-operator.
• [ ] The featureStale Read and Follower Read do not describe at the same level. should be Stale vs Strong, Leader vs Follower.

2. Describe your suggestion or addition.

I will change relative documents.