ticdc: update doc about ddl white list

[wk989898]

First-time contributors' checklist

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

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

Tips for choosing the affected version(s):

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For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
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• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
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• v6.1 (TiDB 6.1 versions)
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• v5.3 (TiDB 5.3 versions)

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[ti-chi-bot]

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[lilin90]

这种“注意”的方式很少使用，是需要类似 TiDB 功能概览 | TiDB 文档中心的脚注吗？是的话，可以参考 https://github.com/pingcap/docs-cn/blob/master/basic-features.md 里的格式。

[ti-chi-bot]

@wk989898: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	f63c207	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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