correct document of using br backup and restore system tables

[Liuxiaozhen12]

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• This PR is translated from: correct document of using br backup and restore system tables docs#6057
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• 3pointer

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[IANTHEREAL]

Rest LGTM

[TomShawn]

/merge

[ti-chi-bot]

@TomShawn: /merge in this pull request requires 2 approval(s).

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/merge

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[TomShawn]

/remove-status LGT1
/status LGT2

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 8110dbc

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6775.