update doc about tiup user

[kaaaaaaang]

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• [ ] v7.3 (TiDB 7.3 versions)
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• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
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[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot[bot]]

@kaaaaaaang: 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	bcb0d0435a4fa10d19f40fabd42dcd74b8355e43	link	true	/test pull-verify

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[ti-chi-bot[bot]]

PR needs rebase.

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[dveeden]

@kaaaaaaang looks like this needs a rebase