update doc

Author: oli2v

docs/content/docs/connectors/flink-sources/postgres-cdc.md

@@ -387,6 +387,13 @@ The following format metadata can be exposed as read-only (VIRTUAL) columns in a
       <td>TIMESTAMP_LTZ(3) NOT NULL</td>
       <td>It indicates the time that the change was made in the database. <br>If the record is read from snapshot of the table instead of the change stream, the value is always 0.</td>
     </tr>
+    <tr>
+      <td>row_kind</td>
+      <td>STRING NOT NULL</td>
+      <td>It indicates the row kind of the changelog,Note: The downstream SQL operator may fail to compare due to this new added column when processing the row retraction if 
+the source operator chooses to output the 'row_kind' column for each record. It is recommended to use this metadata column only in simple synchronization jobs.
+<br>'+I' means INSERT message, '-D' means DELETE message, '-U' means UPDATE_BEFORE message and '+U' means UPDATE_AFTER message.</td>
+    </tr>
   </tbody>
 </table>