sql: clarify description about SET GLOBAL (#22175)

[ti-chi-bot]

This is an automated cherry-pick of #22175

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• This PR is translated from:sql: clarify description about SET GLOBAL docs-cn#21177
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the documentation for the SET GLOBAL SQL statement in TiDB. The primary goal is to enhance clarity regarding how SET GLOBAL affects different types of system variables across the cluster, providing a more accurate comparison to MySQL's behavior and guiding users on how to determine the exact scope of their changes.

Highlights

• Clarified SET GLOBAL behavior: The documentation for SET GLOBAL has been updated to provide a clearer explanation of its behavior in TiDB, specifically differentiating its application to global variables versus instance-level variables.
• Distinction from MySQL: The updated text highlights that unlike MySQL, where SET GLOBAL changes do not propagate to replicas, TiDB's SET GLOBAL can apply to all instances for most system variables, while some are instance-specific.
• Documentation guidance: Users are now advised to consult the system variables documentation, particularly the 'Persists to cluster' attribute, to confirm the scope of changes made with SET GLOBAL for any given variable.

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[gemini-code-assist]

Code Review

This pull request clarifies the behavior of SET GLOBAL in TiDB, which is a valuable improvement for users. The new description is more detailed and accurate. I've suggested a few minor wording and punctuation changes to further improve clarity and align with the documentation style guide. Overall, this is a good clarification.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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