v7.1: modify default value descriptions for enable-partition-separator

[qiancai]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Add information about keeping the default value unchanged for enable-partition-separator

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)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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• Delete files
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• Need modification after applied to another branch
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ran-huang

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[qiancai]

/remove-status LGT1
/status LGT2
/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 98e731d