br: refine description for snapshot backup

[hfxsd]

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What is the related PR or file link(s)?

• This PR is translated from:br: refine description for snapshot backup docs-cn#17636
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-11 11:12:42.860382132 +0000 UTC m=+463116.913694056: ☑️ agreed by Oreoxmt.

[hfxsd]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17861.