Add more description to AUTO_RANDOM (#23498)

[ti-chi-bot]

This is an automated cherry-pick of #23498

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 how signed-bit handling depends on UNSIGNED.
  • Documented that automatically generated signed values use a zero sign bit, while explicitly inserted values are exempt.
  • Added minimum bit requirements and separate sizing formulas for signed and unsigned auto-increment columns.

[coderabbitai]

📝 Walkthrough

Walkthrough

The AUTO_RANDOM documentation now clarifies signed-bit behavior, positivity for implicitly generated values, explicit-value exceptions, and minimum auto-increment-bit formulas for signed and unsigned columns.

Changes

Auto-random bit-layout documentation

Layer / File(s)	Summary
Clarify auto-random bit requirements auto-random.md	Documents signed-bit behavior based on UNSIGNED, positivity for implicitly generated values, the exception for explicit values, and the minimum auto-increment-bit formulas.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Possibly related PRs

• pingcap/docs#23498: Updates the same documentation with related signed/unsigned bit-layout clarifications.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation update for AUTO_RANDOM.
Description check	✅ Passed	The description covers the changes, affected versions, related links, and required checklist sections.
Linked Issues check	✅ Passed	The documentation adds the requested AUTO_RANDOM positivity behavior and minimum auto-increment-bit limitation from issue #23404.
Out of Scope Changes check	✅ Passed	The changes are limited to clarifying AUTO_RANDOM documentation requirements from issue #23404.
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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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between b5b0a64 and 83d22fd.

📒 Files selected for processing (1)

• auto-random.md

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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