ttl: document index scans for TTL jobs

[YangKeao]

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

Translate the TTL index scan documentation from pingcap/docs#23813.

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

• 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): ttl: enhance TTL scan with secondary index tidb#68397
• This PR is translated from: ttl: document index scans for TTL jobs docs#23813
• Other reference link(s): N/A

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

• 文档
  • 新增系统变量 tidb_ttl_enable_index_scan，默认值为 ON，用于控制 TTL 任务是否使用符合条件的索引扫描。
  • 补充 TTL 过期行索引扫描的适用条件及索引要求。
  • 说明关闭该变量或无可用索引时，将回退为按表键顺序扫描。
  • 补充 mysql.tidb_ttl_task 中 scan_index_id 字段的含义，用于识别任务采用的扫描方式。

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📝 Walkthrough

Walkthrough

新增 TTL 索引扫描系统变量文档，说明索引使用条件、关闭变量或无匹配索引时的回退行为，并补充 TTL 任务表中的 scan_index_id 字段说明。

Changes

TTL 索引扫描文档

Layer / File(s)	Summary
系统变量定义与引用 system-variables.md, system-variable-reference.md	新增 tidb_ttl_enable_index_scan。文档说明其全局作用域、集群持久化属性、默认值和关闭后的扫描行为，并加入参考链接。
TTL 索引扫描行为 time-to-live.md	新增索引扫描条件、回退规则和优化器限制说明。补充 mysql.tidb_ttl_task.scan_index_id 的字段含义。

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

Suggested reviewers: qiancai, oreoxmt, ti-chi-bot

Merge Risk: 🔵 Low · up to 5cf35

该文档新增 TTL 索引扫描规则，但索引类型术语存在误译和遗漏，可能导致用户错误判断某个索引是否会被 TTL 使用；应修正后合并。

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题简洁明确，准确概括了为 TTL 任务补充索引扫描文档的主要变更。
Description check	✅ Passed	描述已填写变更内容、适用版本、相关 PR 和 AI agent involvement。必需信息完整，未发现影响审核的信息缺失。
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.

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ℹ️ Review info

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📥 Commits

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📒 Files selected for processing (2)

• system-variables.md
• time-to-live.md

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