cdc: add description about canal-json content-compatible (#15568)

ticdc/ticdc-canal-json.md

@@ -306,6 +306,13 @@ The way that TiCDC implements the Canal-JSON data format, including the `Update`
 | Event of `Update` Type  | By default, the `old` field contains all the column data. When `only_output_updated_columns` is `true`, the `old` field contains only the modified column data.  | The `old` field contains only the modified column data    |
 | `mysqlType` field  | For types with parameters, it does not contain the information of the type parameter                                                         | For types with parameters, it contains the full information of the type parameter    |
 
+### Compatibility with the official Canal
+
+Starting from v6.5.6, TiCDC Canal-JSON supports compatibility with the data format of the official Canal. When creating a changefeed, you can set `content-compatible=true` in `sink-uri` to enable this feature. In this mode, TiCDC outputs Canal-JSON format data that is compatible with the official Canal. The specific changes are as follows:
+
+* The `mysqlType` field contains the full information of the type parameter for each type.
+* An Event of `Update` Type only outputs the modified column data.
+
 ### Event of `Update` Type
 
 For an Event of `Update` Type: