sql: document unsupported GRANT PROXY in MySQL compatibility (#23742)

[ti-chi-bot]

This is an automated cherry-pick of #23742

What is changed, added or deleted? (Required)

• Add a 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.

• 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:
• 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

• Documentation
  • Clarified that TiDB does not support MySQL’s GRANT PROXY statement.
  • Documented support starting in TiDB v8.5.6 for granting and revoking SELECT, INSERT, UPDATE, and REFERENCES privileges on specific columns.
  • Added a link to the column-level privilege management documentation.

[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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Reviewing files that changed from the base of the PR and between b40a15e and 7d44c24.

📒 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

The grant privilege compatibility documentation now states that TiDB does not support GRANT PROXY.

Changes

Grant privilege compatibility documentation

Layer / File(s)	Summary
Update compatibility notes sql-statements/sql-statement-grant-privileges.md	Adds a MySQL compatibility note for unsupported GRANT PROXY syntax.

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

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to 7d44c

This change only clarifies that TiDB does not support GRANT PROXY and does not alter runtime behavior. No actionable product risk is evidenced.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the SQL documentation change and the unsupported GRANT PROXY behavior in MySQL compatibility.
Description check	✅ Passed	The description covers the required change, affected version, reference link, and AI involvement. It is sufficiently complete for this documentation-only cherry-pick.
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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[coderabbitai]

Actionable comments posted: 1

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

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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