v7.1.6: ticdc: add description for cdc behaviour change (#17363)

[ti-chi-bot]

This is an automated cherry-pick of #17363

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PR needs rebase.

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[qiancai]

Suggested change

	从 v7.1.6 开始，当使用 MySQL Sink 时，TiCDC 的任意节点在收到某张表的同步任务请求并开始向下游同步数据之前，会从 PD 获取当前的时间戳 `thresholdTS`，并根据时间戳的值决定是否拆分 `UPDATE` 事件：
	从 v6.5.10 和 v7.1.6 开始，当使用 MySQL Sink 时，TiCDC 的任意节点在收到某张表的同步任务请求并开始向下游同步数据之前，会从 PD 获取当前的时间戳 `thresholdTS`，并根据时间戳的值决定是否拆分 `UPDATE` 事件：

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[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch master: #19141.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #19142.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19143.