Document IMPORT INTO restriction for TTL-enabled tables

[lilin90]

What is changed, added or deleted? (Required)

Document that IMPORT INTO does not support importing data into a table with TTL enabled.

The restrictions list already covered temporary and cached tables but omitted TTL-enabled tables. As described in pingcap/tidb#70426, an asynchronous TTL job can delete rows while an import is running, which can cause the final checksum verification to fail. This update closes that documentation gap so users can avoid running IMPORT INTO against an unsupported target table.

Validation: ./scripts/markdownlint sql-statements/sql-statement-import-into.md and git diff --check.

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

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

• This PR is translated from: Document IMPORT INTO restriction for TTL-enabled tables docs#23507
• Other reference link(s):
  • Issue: IMPORT INTO can fail with checksum mismatch when target table has TTL enabled tidb#70426
  • PR: importinto: forbid importing into TTL-enabled tables tidb#70427

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
  • 更新 IMPORT INTO 使用限制：不支持将数据导入启用 TTL 的表。
  • 保持临时表和缓存表的现有限制不变。

[github-actions]

Auto-sync completed successfully

Source PR: pingcap/docs#23507
Target PR: #21881

Chinese documentation has been updated based on English documentation changes.

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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 26f202b and 1093f41.

📒 Files selected for processing (1)

• sql-statements/sql-statement-import-into.md

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📝 Walkthrough

Walkthrough

Changes

IMPORT INTO 使用限制

Layer / File(s)	Summary
新增 TTL 表限制 sql-statements/sql-statement-import-into.md	文档规定启用了 TTL 的表不得作为 IMPORT INTO 的目标表。临时表和缓存表的限制保持不变。

Estimated code review effort: 1 (简单) | ~3 分钟

Possibly related issues

• pingcap/tidb#70426：该变更直接补充了该 issue 中关于 TTL 表不能作为 IMPORT INTO 目标表的限制。

Suggested reviewers: d3hunter

🚥 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.
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，即记录 IMPORT INTO 对启用 TTL 表的限制。
Description check	✅ Passed	描述完整说明了变更内容、原因、适用版本、相关链接和验证方式，且已填写必要版本选项。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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