Add config output-handle-key/output-old-value doc for ticdc (#16812) (#…

…16867)

ticdc/ticdc-changefeed-config.md

@@ -193,6 +193,9 @@ enable-partition-separator = true
 # include-commit-ts = false
 # The encoding method of binary data, which can be 'base64' or 'hex'. The default value is 'base64'.
 # binary-encoding-method = 'base64'
+# Whether to output the value before the row data changes. The default value is false. 
+# When it is enabled, the UPDATE event will output two rows of data: the first row is a DELETE event that outputs the data before the change; the second row is an INSERT event that outputs the changed data.
+# output-old-value = false
 
 # Specifies the replication consistency configurations for a changefeed when using the redo log. For more information, see https://docs.pingcap.com/tidb/stable/ticdc-sink-to-mysql#eventually-consistent-replication-in-disaster-scenarios.
 # Note: The consistency-related configuration items only take effect when the downstream is a database and the redo log feature is enabled.