cdc: add description about checksum v2

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This is planned to be release since v8.4.0

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[qiancai]

Rest LGTM

[qiancai]

Suggested change

	在 v8.4.0 之前，TiDB 和 TiCDC 采用 Checksum v1 进行 Checksum 计算和校验。
	在 v8.4.0 之前，TiDB 和 TiCDC 采用 Checksum v1 算法进行 Checksum 计算和校验。

[qiancai]

Suggested change

	在启用单行数据 Checksum 正确性校验功能后，TiDB 使用 CRC32 算法计算该行数据的 Checksum 值，并将其一并写入 TiKV。TiCDC 从 TiKV 读取数据，根据相同的算法重新计算 Checksum，如果该值与 TiDB 写入的值相同，则可以证明数据在 TiDB 至 TiCDC 的传输过程中是正确的。
	在启用单行数据 Checksum 正确性校验功能后，TiDB 会使用 CRC32 算法计算该行数据的 Checksum 值，并将这个值与该行数据一并存储在 TiKV 中。随后，TiCDC 从 TiKV 读取这些数据，并使用相同的算法重新计算 Checksum，如果得到的 Checksum 值与 TiDB 写入的 Checksum 值相同，则表明数据在从 TiDB 到 TiCDC 的传输过程中是正确的。

[qiancai]

Suggested change

	TiCDC 将数据编码成特定格式并发送至 Kafka。Kafka Consumer 读取数据后，可以使用与 TiDB 相同的算法计算得到新的 Checksum，将此值与数据中携带的 Checksum 值进行比较，若二者一致，则可证明从 TiCDC 至 Kafka Consumer 的传输链路上的数据是正确的。
	TiCDC 将数据编码成特定格式并发送至 Kafka。Kafka Consumer 读取数据后，可以使用与 TiDB 相同的 CRC32 算法计算得到新的 Checksum，将此值与数据中携带的 Checksum 值进行比较，若二者一致，则表明从 TiCDC 到 Kafka Consumer 的传输链路上的数据是正确的。

[qiancai]

Suggested change

	从 v8.4.0 开始，TiDB 和 TiCDC 引入 Checksum V2 算法，解决了 Checksum V1 在执行 Add Column 或 Drop Column 后无法正确校验 Update 或 Delete 事件中 Old Value 数据的问题。
	从 v8.4.0 开始，TiDB 和 TiCDC 引入 Checksum V2 算法，解决了 Checksum V1 在执行 `ADD COLUMN` 或 `DROP COLUMN` 后无法正确校验 Update 或 Delete 事件中 Old Value 数据的问题。

[qiancai]

Rest LGTM

[qiancai]

Rest LGTM

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[LGTM Timeline notifier]

Timeline:

• 2024-09-23 08:01:17.390506097 +0000 UTC m=+1466547.130930037: ☑️ agreed by Oreoxmt.
• 2024-09-24 01:33:46.801885739 +0000 UTC m=+1529696.542309678: ☑️ agreed by qiancai.