br: add more info about the br log structure (#20443)

[ti-chi-bot]

This is an automated cherry-pick of #20443

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What is changed, added or deleted? (Required)

让 BR log backup 的文件结构更加友好一些。
Made the file structure paragraph in the br_log_architecture.md more friendly.

同时解决了 pingcap/docs-cn#19767 中提到的 min_restored_ts 和现行惯用词不同的问题。
Meanwhile, solved the problem that min_resolved_ts is inconsistent with customary usage mentioned at pingcap/docs-cn#19767.

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• This PR is translated from: 19824
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