cloud: modify private endpoint doc according to feedback

[TomShawn]

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cloud: modify private endpoint doc according to feedback

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

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• master (the latest development version)
• v6.3 (TiDB 6.3 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
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• v5.0 (TiDB 5.0 versions)

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