v7.1.6: ticdc: add description for cdc behaviour change (#17530) (#17672)

[ti-chi-bot]

This is an automated cherry-pick of #17672

This is an automated cherry-pick of #17530

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

What is changed, added or deleted? (Required)

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

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For details, see tips for choosing the affected versions.

• [x] master (the latest development version)
• [x] v8.1 (TiDB 8.1 versions)
• [ ] v8.0 (TiDB 8.0 versions)
• [ ] v7.6 (TiDB 7.6 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)
• [ ] v5.2 (TiDB 5.2 versions)
• [ ] v5.1 (TiDB 5.1 versions)

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

• This PR is translated from: https://github.com/pingcap/docs-cn/pull/17363
• 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

[qiancai]

/approve

[ti-chi-bot[bot]]

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