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

[ti-chi-bot]

This is an automated cherry-pick of #19156

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• [x] master (the latest development version)
• [x] v8.4 (TiDB 8.4 versions)
• [x] v8.3 (TiDB 8.3 versions)
• [x] v8.2 (TiDB 8.2 versions)
• [x] v8.1 (TiDB 8.1 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [x] v7.1 (TiDB 7.1 versions)
• [x] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions)

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

• This PR is translated from:https://github.com/pingcap/docs-cn/pull/18783
• 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

[ti-chi-bot[bot]]

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[ti-chi-bot[bot]]

@ti-chi-bot: 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	a589f93e72d2436ccb56c80ab26a47c8cc9254dc	link	true	/test pull-verify

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