cdc: make mq encoding protocol related more clear (#19156)

[ti-chi-bot]

This is an automated cherry-pick of #19156

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• [ ] v6.1 (TiDB 6.1 versions)
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• This PR is translated from:https://github.com/pingcap/docs-cn/pull/18783
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• [ ] Delete files
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• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

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