Fix the desciption about cluster index and pre split region on auto random column (#20056)

[ti-chi-bot]

This is an automated cherry-pick of #20056

First-time contributors' checklist

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What is changed, added or deleted? (Required)

Modify the limitation of cluster index. PRE_SPLIT_REGION is supported for auto random column.

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

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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)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 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)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: Fix the desciption about cluster index and pre split region on auto random column docs#20551
• Other reference link(s):

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

[Oreoxmt]

/approve

[ti-chi-bot]

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