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

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• 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.

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/approve

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