pd/tls: add upgrade tls description (#2612)

[ti-chi-bot]

This is an automated cherry-pick of #2612

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)

• master (the latest development version)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)
• 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): pd: support schema change by tls tidb-operator#5708

[Oreoxmt]

/approve

[ti-chi-bot]

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