br: add document for backup compression (#14834)

[ti-chi-bot]

This is an automated cherry-pick of #14834

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What is changed, added or deleted? (Required)

add comments about backup config: compression && compression-level

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• Delete files
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[ran-huang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ran-huang

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[RobertCheng-956]

/translation done

[ti-chi-bot]

@RobertCheng-956: These labels are not set on the issue: translation/welcom.

In response to this:

/remove-translation welcom

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[ti-chi-bot]

@RobertCheng-956: These labels are not set on the issue: translation/welcom.

In response to this:

/remove-translation welcom

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[RobertCheng-956]

/remove-translation done