pd/tls: add upgrade tls description

[HuSharp]

First-time contributors' checklist

• [ ] I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added, or deleted? (Required)

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

• [x] master (the latest development version)
• [x] v1.6 (TiDB Operator 1.6 versions)
• [x] v1.5 (TiDB Operator 1.5 versions)
• [x] v1.4 (TiDB Operator 1.4 versions)
• [x] v1.3 (TiDB Operator 1.3 versions)
• [x] v1.2 (TiDB Operator 1.2 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): https://github.com/pingcap/tidb-operator/pull/5708