pd/tls: add upgrade tls description

[hfxsd]

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• 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)

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

• This PR is translated from: pd/tls: add upgrade tls description #2612
• Other reference link(s):

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-18 02:14:50.381954397 +0000 UTC m=+927532.778176455: ☑️ agreed by Oreoxmt.

[hfxsd]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.4: #2711.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.5: #2712.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.6: #2713.