system variables: document tidb_enable_adaptive_limit_scan

[solotzg]

What is changed, added, or deleted? (Required)

Document the new tidb_enable_adaptive_limit_scan system variable introduced by pingcap/tidb#70252, including its scope, persistence, experimental status, default values for fresh and upgraded clusters, and supported query plans.

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

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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)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)?

• Related code change PR links (if applicable): executor: add adaptive admission control for ordered LIMIT scans tidb#70252
• This PR is translated from:
• Other reference link(s):

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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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 0f9f2f8 and c7fb1f7.

📒 Files selected for processing (2)

• system-variable-reference.md
• system-variables.md

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📝 Walkthrough

Walkthrough

Changes

自适应 LIMIT 扫描系统变量

Layer / File(s)	Summary
系统变量定义与索引 system-variables.md, system-variable-reference.md	新增 tidb_enable_adaptive_limit_scan。文档记录其作用域、持久化属性、类型、默认值、适用计划和版本信息。系统变量索引新增对应链接。

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to 9c0d7

This change documents and indexes the adaptive LIMIT scan variable; no merge-blocking production risk is identified in the supplied current-head evidence.

🚥 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：为系统变量 tidb_enable_adaptive_limit_scan 添加文档。标题简洁且与主要改动一致。
Description check	✅ Passed	描述包含变更内容、适用版本、关联代码 PR、AI agent involvement 和变更类型检查项。必填信息基本完整，且与 PR 改动一致。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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