sql: document unsupported GRANT PROXY in MySQL compatibility (#21924)

[ti-chi-bot]

This is an automated cherry-pick of #21924

What is changed, added or deleted? (Required)

• Add the corresponding Chinese MySQL compatibility note to sql-statements/sql-statement-grant-privileges.md to clarify that TiDB does not support GRANT PROXY.
• This update is scoped to release-8.5 because it documents the currently published behavior verified in TiDB Cloud Starter.

Which TiDB version(s) do your changes apply to? (Required)

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 (in Chinese).

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 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: sql: document unsupported GRANT PROXY in MySQL compatibility docs#23742
• Other reference link(s): https://docs.pingcap.com/tidb/stable/sql-statement-grant-privileges/

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
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• 文档更新
  • 更新 GRANT <privileges> 的 MySQL 兼容性说明。
  • 补充 TiDB 自 v8.5.6 起支持列级权限管理，可针对特定表列授予或回收 SELECT、INSERT、UPDATE 和 REFERENCES 权限。
  • 说明 TiDB 不支持 MySQL 的 GRANT PROXY。

[ti-chi-bot]

@doc-claw-bot This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 482a58c and ce7af34.

📒 Files selected for processing (1)

• sql-statements/sql-statement-grant-privileges.md

💤 Files with no reviewable changes (1)

• sql-statements/sql-statement-grant-privileges.md

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📝 Walkthrough

Walkthrough

更新 GRANT <privileges> 文档的 MySQL 兼容性章节，说明 TiDB 不支持 GRANT PROXY。

Changes

GRANT 权限兼容性

Layer / File(s)	Summary
更新 MySQL 兼容性说明 sql-statements/sql-statement-grant-privileges.md	新增 TiDB 不支持 GRANT PROXY 的说明。

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

Merge Risk: ⚪ Minimal · up to ce7af

This update only documents that TiDB does not support GRANT PROXY and does not change product behavior; resolve the noted merge conflicts before merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了主要变更：在 MySQL 兼容性文档中说明不支持 GRANT PROXY。
Description check	✅ Passed	描述包含变更内容、适用版本、相关 PR 和参考链接，并说明了 v8.5 范围及 AI 参与情况。整体符合模板要求。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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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Actionable comments posted: 1

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• sql-statements/sql-statement-grant-privileges.md

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[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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