tidb: enable global memory arbitration in priority mode by default

[solotzg]

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• Other reference link(s): *: enable global memory arbitration in priority mode by default tidb#69638

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Summary by CodeRabbit

• 文档
  • 更新系统变量 tidb_mem_arbitrator_mode 的默认值说明：从 disable 调整为 priority。
  • 保留现有可选值（disable、standard、priority）及各模式含义说明不变。

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📝 Walkthrough

Walkthrough

文档将系统变量 tidb_mem_arbitrator_mode 的默认值从 disable 更新为 priority，并移除 disable 枚举项的默认值标注。

Changes

系统变量文档

Layer / File(s)	Summary
更新内存仲裁模式默认值 system-variables.md	将 tidb_mem_arbitrator_mode 的默认值更新为 priority，并移除 disable 条目的默认值标注；可选值及模式描述保持不变。

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

Suggested reviewers: qiancai, lilin90, guo-shaoge

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	模板结构基本齐全，但“What's changed”必填内容仍为空，缺少变更说明和原因。	补充“What is changed, added or deleted?”章节，说明修改了哪些文档、默认值为何改为 priority，以及影响范围。

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	标题与变更一致，准确概括了将内存仲裁默认模式改为 priority。

✨ Finishing Touches

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• Commit unit tests in branch solotzg-patch-1

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Actionable comments posted: 1

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Actionable comments posted: 1

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• system-variables.md

[qiancai]

改了默认值后，相当于这个功能在所有环境都默认开启了，那前面的 “该变量控制的功能为实验特性，不建议在生产环境中使用” 还需要保留吗，有些矛盾