tikv: improve the note for s3-multi-part-size (#23362)

[ti-chi-bot]

This is an automated cherry-pick of #23362

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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.

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Summary by CodeRabbit

• Documentation
  • Updated guidance for the backup.s3-multi-part-size setting.
  • Added recommendations for careful configuration and warnings about upload failures or timeouts when using values that are too large on unstable networks.

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📥 Commits

Reviewing files that changed from the base of the PR and between 24494e7 and d7b539b.

📒 Files selected for processing (1)

• tikv-configuration-file.md

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📝 Walkthrough

Walkthrough

Updates the backup.s3-multi-part-size documentation to provide cautious configuration guidance from TiDB v6.1.1 onward and warn about multipart upload failures or timeouts with oversized parts on unstable networks.

Changes

S3 multipart size documentation

Layer / File(s)	Summary
Update multipart size guidance tikv-configuration-file.md	Replaces the previous deprecation note with guidance on S3 rate-limiting failures, cautious configuration, and oversized multipart upload risks.

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

Possibly related PRs

• pingcap/docs#23362: Updates the same configuration note with effectively identical guidance.

Suggested reviewers: qiancai, mittalrishabh, lilin90

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the doc note update for s3-multi-part-size.
Description check	✅ Passed	The description mostly matches the template and includes scope, versions, and cherry-pick context, though the change-summary section is sparse.
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.

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