cloud: update doc for a toggle that disables public ip

[ran-huang]

What is changed, added or deleted? (Required)

Add doc for DM-6936

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• v6.4 (TiDB 6.4 versions)
• v6.3 (TiDB 6.3 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
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[ti-chi-bot]

@ran-huang: PR needs rebase.

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[ti-chi-bot]

@ran-huang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	d617cad	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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