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)

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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)

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• 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
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• Might cause conflicts after applied to another branch

Summary by CodeRabbit

文档

• 更新 AUTO_RANDOM 位结构说明，明确符号位取决于 UNSIGNED 属性。
• 说明隐式分配值始终为正数，且有符号列的符号位固定为 0。
• 补充自增位长度至少为 27 位的约束，并保留原有保留位说明。

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[qiancai]

/approve

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