cdc: add description about the tidb extension field

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[hfxsd]

Suggested change

	_tidb 即 TiDB 扩展字段中的子字段说明：
	| 字段 | 类型 | 说明 |
	|:-------------------|:-------|:-------------------------------------------------------------------------|
	| commitTs | Number | 产生事件变更的事务提交使用的 TSO。 设置于 DDL 和 DML 事件 |
	| watermarkTs | Number | TiCDC 周期性发送的 watermark ts，其类型也是 TSO。设置于 Watermark 事件|
	| tableId | Number | 事件所属表的 ID。设置于 DML 事件 |
	| partitionId | Number | 事件如果来自于分区表，消息会携带该字段，表示所属分区的 ID。设置于 DML 事件 |
	| onlyHandleKey | Bool | 当前事件仅包含 handle key 部分内容。设置于 DML 事件 |
	| claimCheckLocation | String | 当前事件含有 claim check location。设置于 DML 事件 |
	tableId 和 partitionId 字段从 xxx 版本开始引入，主要目的是为了应对分区表场景下，消费者可以通过 partitionId 得知当前事件所属的表分区，以及关联的分区表。
	_tidb 即 TiDB 扩展字段，其子字段说明如下：
	| 字段 | 类型 | 说明 |
	|:-------------------|:-------|:-------------------------------------------------------------------------|
	| commitTs | Number | 产生事件变更的事务提交使用的 TSO。设置于 DDL 和 DML 事件 |
	| watermarkTs | Number | TiCDC 周期性发送的 Watermark Ts，其类型也是 TSO。设置于 Watermark 事件|
	| tableId | Number | 事件所属表的 ID。设置于 DML 事件 |
	| partitionId | Number | 事件如果来自于分区表，消息会携带该字段，表示所属分区的 ID。设置于 DML 事件 |
	| onlyHandleKey | Bool | 当前事件仅包含 handle key 部分内容。设置于 DML 事件 |
	| claimCheckLocation | String | 当前事件含有 claim check location。设置于 DML 事件 |
	`tableId` 和 `partitionId` 字段从 xxx 版本开始引入，主要目的是为了应对分区表场景下，消费者可以通过 `partitionId` 得知当前事件所属的表分区，以及关联的分区表。