br: add more info about the br log structure

[YuJuncen]

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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。
同时解决了 #19767 中提到的 min_restored_ts 和现行惯用词不同的问题。

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[BornChanger]

这一行是多的吧？

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