Add more description to AUTO_RANDOM

[qiancai]

What is changed, added or deleted? (Required)

close #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: Add more description to AUTO_RANDOM docs-cn#21863
• 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

• Documentation
  • Clarified signed and unsigned column behavior in AUTO_RANDOM bit layouts.
  • Documented that implicitly generated values are always positive with a zero signed bit.
  • Added the minimum 27-bit requirement for auto-increment bits.

[github-actions]

Auto-sync completed successfully

Source PR: pingcap/docs-cn#21863
Target PR: #23498

English documentation has been updated based on Chinese documentation changes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

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Run ID: 78bcdcec-bf72-403c-9780-78d34df26582

📥 Commits

Reviewing files that changed from the base of the PR and between 1e0eb57 and 6ac0372.

📒 Files selected for processing (1)

• auto-random.md

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📝 Walkthrough

Walkthrough

The AUTO_RANDOM documentation now distinguishes signed and unsigned columns, states that implicit values are always positive, and defines the minimum auto-increment-bit requirements.

Changes

AUTO_RANDOM documentation

Layer / File(s)	Summary
Document bit-layout constraints auto-random.md	Clarifies signed-bit behavior, implicit positive values, and minimum auto-increment-bit requirements for signed and unsigned columns.

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the documentation change to AUTO_RANDOM.
Description check	✅ Passed	The description covers the required change, affected versions, related issue, translated PR, and checklist fields.
Linked Issues check	✅ Passed	The changes address issue #23404 by documenting positive default values and the 27-bit auto-increment limitation.
Out of Scope Changes check	✅ Passed	The changes are limited to the requested AUTO_RANDOM documentation clarifications.
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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[qiancai]

/cc @djshow832

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@djshow832: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[qiancai]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #23499.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #23500.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #23501.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #23502.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #23503.