tikv: improve the note for s3-multi-part-size (#21841)

[ti-chi-bot]

This is an automated cherry-pick of #21841

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What is changed, added or deleted? (Required)

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In real-world scenarios, a small percentage of customers still exceed their S3 API quotas, making this configuration still valuable. Since there is no other workaround and the configuration will not be removed, this update replaces the word "deprecated" with "configure with caution." This aims to reduce the hesitation or mental burden for customers who genuinely need to use it to resolve their issues.

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from: tikv: improve the note for s3-multi-part-size docs#23362
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
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Summary by CodeRabbit

• 文档
  • 更新 backup.s3-multi-part-size 配置项说明，补充 S3 限流可能导致备份失败的相关信息。
  • 增加风险提示：自 TiDB v6.1.1 起请谨慎设置该参数；分块过大时，网络波动可能导致上传失败或超时。
  • 保留原有分块大小说明及默认值信息。

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 27edfca2-8a77-4043-abce-b8049b28ac7b

📥 Commits

Reviewing files that changed from the base of the PR and between 2ba27b6 and 10c536e.

📒 Files selected for processing (1)

• tikv-configuration-file.md

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📝 Walkthrough

Walkthrough

更新 backup.s3-multi-part-size 配置项文档，替换原有废弃说明，并补充 TiDB v6.1.1 起的设置注意事项及网络抖动风险。

Changes

S3 配置文档

Layer / File(s)	Summary
S3 分块大小说明更新 tikv-configuration-file.md	更新 backup.s3-multi-part-size 的版本说明，补充过大分块在网络抖动时可能导致上传失败或超时的提示。

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• pingcap/docs-cn#21841：同样更新 tikv-configuration-file.md 中 backup.s3-multi-part-size 的相关说明。

Suggested labels: translation/from-docs, needs-cherry-pick-master, approved

Suggested reviewers: qiancai, lilin90

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	缺少必填的“What is changed, added or deleted?”内容，模板要求的变更说明未完整填写。	补充“ What is changed, added or deleted?”部分，明确说明改了什么、为什么改，并保持与模板字段对应。

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题与实际变更一致，准确概括了对 s3-multi-part-size 说明的更新。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[qiancai]

/approve

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