Document tidb_query_cop_store_limit variable

[solotzg]

Added documentation for the tidb_query_cop_store_limit system variable, including its scope, persistence, type, default value, range, and behavior.

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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): distsql, kv: add query-level per-store cop request limiter tidb#69360

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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Summary by CodeRabbit

• 文档
  • 新增系统变量 tidb_query_cop_store_limit 的说明。
  • 补充其 SESSION/GLOBAL 作用域、默认值 15、取值范围 [0, 256]、集群持久化及 SET_VAR Hint 支持。
  • 说明该变量对 TiKV Coprocessor 请求并发尝试数的限制，以及设置为 0 时禁用限制。
  • 明确其与扫描并发度设置的区别，以及不影响 TiFlash 请求。

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📝 Walkthrough

Walkthrough

文档新增系统变量 tidb_query_cop_store_limit。条目说明其作用域、默认值、取值范围、持久化方式，以及对 TiKV Coprocessor 请求的并发限制行为。

Changes

查询 Coprocessor 并发限制

Layer / File(s)	Summary
新增系统变量文档条目 system-variables.md	新增 tidb_query_cop_store_limit 的配置属性和行为说明，包括 SESSION、GLOBAL 作用域，默认值 15，取值范围 [0, 256]，SET_VAR Hint 支持和集群持久化。文档说明该变量仅限制 TiKV Coprocessor 请求，并区分 tidb_distsql_scan_concurrency 的作用。

Estimated code review effort: 1 (Trivial) | ~3 minutes

Merge Risk: 🔵 Low · up to 0edfc

The documentation adds the variable but currently omits the special behavior of setting it to 0, and the first supported release marker still needs confirmation. The PR is mergeable with explicit owner follow-up to correct the boundary-value description and verify the version marker.

Suggested reviewers: windtalker, oreoxmt, qiancai

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	标题准确概括了本次变更，即新增 tidb_query_cop_store_limit 系统变量文档，内容简洁明确。
Description check	✅ Passed	描述包含变更内容、适用版本、相关 PR、CLA 状态、AI agent 参与情况和其他变更选项。开头说明覆盖了必填的变更内容，因此整体信息完整。

Full details: Docstring Coverage

Explanation

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 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch solotzg-patch-2

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[windtalker]

lgtm

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