ticdc: add description about safe-mode arguments (#20314) (#20326)

Author: ti-chi-bot

ticdc/ticdc-sink-to-mysql.md

@@ -70,6 +70,7 @@ The following are descriptions of sink URI parameters and parameter values that
 | `ssl-key` | The path of the certificate key file needed to connect to the downstream MySQL instance (optional). |
 | `time-zone` | The time zone used when connecting to the downstream MySQL instance, which is effective since v4.0.8. This is an optional parameter. If this parameter is not specified, the time zone of TiCDC service processes is used. If this parameter is set to an empty value, such as `time-zone=""`, no time zone is specified when TiCDC connects to the downstream MySQL instance and the default time zone of the downstream is used. |
 | `transaction-atomicity`  |  The atomicity level of a transaction. This is an optional parameter, with the default value of `none`. When the value is `table`, TiCDC ensures the atomicity of a single-table transaction. When the value is `none`, TiCDC splits the single-table transaction.  |
+| `safe-mode` | Specifies how TiCDC handles `INSERT` and `UPDATE` statements when replicating data to the downstream. When it is `true`, TiCDC converts all upstream `INSERT` statements to `REPLACE INTO` statements, and all `UPDATE` statements to `DELETE` + `REPLACE INTO` statements. Before v6.1.3, the default value of this parameter is `true`. Starting from v6.1.3, the default value is changed to `false`. When TiCDC starts, it obtains a current timestamp `ThresholdTs`. For `INSERT` and `UPDATE` statements with `CommitTs` less than `ThresholdTs`, TiCDC converts them to `REPLACE INTO` statements and `DELETE` + `REPLACE INTO` statements respectively. For `INSERT` and `UPDATE` statements with `CommitTs` greater than or equal to `ThresholdTs`, `INSERT` statements are directly replicated to the downstream, while the behavior of `UPDATE` statements follows the [TiCDC Behavior in Splitting UPDATE Events](/ticdc/ticdc-split-update-behavior.md). |
 
 To encode the database password in the sink URI using Base64, use the following command: