Add documentation on how to modify gcttl by tiup (#6471)

[ti-chi-bot]

This is an automated cherry-pick of #6471

Add documentation on how to modify gcttl by tiup.
#1825

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• v5.1 (TiDB 5.1 versions)
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: 44ad836