Clarify temporary PD lifecycle in pd-recover

[JmPotato]

What is changed, added or deleted? (Required)

补充 pd-recover 从存活 PD 节点重建集群时的步骤依赖说明：

• 第 2 步启动的 --force-new-cluster PD 是临时单节点 PD，需要保持运行，供第 3 步 pd-recover 连接。
• 第 3 步需要另开终端窗口执行。
• 第 4 步明确需要停止第 2 步的临时 PD，并不带 --force-new-cluster 正常重启。
• 第 5 步明确在 PD 正常启动并提供服务后再扩容 PD 和启动其他组件。

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

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Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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