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。
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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[lilin90]

/lgtm

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-03-04 09:26:16.929502736 +0000 UTC m=+347290.058422481: ☑️ agreed by lilin90.
• 2025-03-04 10:09:28.875550453 +0000 UTC m=+349882.004470199: ☑️ agreed by qiancai.

[lilin90]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #20455.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #20456.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20457.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #20458.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #20459.