Add more description to AUTO_RANDOM (#21863)

[ti-chi-bot]

This is an automated cherry-pick of #21863

What is changed, added or deleted? (Required)

close pingcap/docs#23404

Add 2 more descriptions:

• The auto_random value is always positive unless explicitly inserted.
• The auto increment bits must not be less than 27: https://github.com/pingcap/tidb/blob/release-8.5/pkg/meta/autoid/autoid.go#L86

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

Tips for choosing the affected version(s):

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)?

• 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.

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 AUTO_RANDOM 位结构说明，明确 UNSIGNED 属性对符号位的影响。
  • 补充隐式生成值始终为正数及有符号列符号位为 0 的规则。
  • 说明自增位至少为 27 位，并完善有符号、无符号列的计算条件。

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: d96aa5b2-b37a-4618-a1fc-6af0fd979073

📥 Commits

Reviewing files that changed from the base of the PR and between a57b643 and fe544aa.

📒 Files selected for processing (1)

• auto-random.md

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📝 Walkthrough

Walkthrough

更新 AUTO_RANDOM 的位结构说明，补充符号位规则、自增位至少 27 位的约束，以及有符号列和无符号列的计算条件。

Changes

AUTO_RANDOM 文档更新

Layer / File(s)	Summary
位规则与数量约束 auto-random.md	说明符号位由 UNSIGNED 属性决定，隐式生成值始终为正数，并补充自增位至少 27 位及其计算条件。

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

Possibly related PRs

• pingcap/docs-cn#21863：同样更新 auto-random.md，说明 AUTO_RANDOM 的符号行为和最小自增位数。

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	变更补充了默认正值说明，但将位数描述为至少 27 位，未严格对应问题中的大于 27 位要求。	请确认约束边界；如果要求严格大于 27 位，请同步修改文档表述和相关说明。

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题明确说明了对 AUTO_RANDOM 文档进行补充，且与主要变更一致。
Description check	✅ Passed	描述包含变更内容、适用版本和关联问题，已满足模板的主要要求。
Out of Scope Changes check	✅ Passed	变更仅涉及 AUTO_RANDOM 文档说明，未发现与关联问题无关的内容。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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