ticdc: add example of how to show bdr role (pingcap#19238)

Author: qiancai

ticdc/ticdc-bidirectional-replication.md

@@ -99,6 +99,20 @@ In short, in BDR mode, TiCDC only replicates replicable DDLs in the PRIMARY clus
 ### Replication scenarios of replicable DDLs
 
 1. Choose a TiDB cluster and execute `ADMIN SET BDR ROLE PRIMARY` to set it as the primary cluster.
+
+    ```sql
+    ADMIN SET BDR ROLE PRIMARY;
+    Query OK, 0 rows affected
+    Time: 0.003s
+
+    ADMIN SHOW BDR ROLE;
+    +----------+
+    | BDR_ROLE |
+    +----------+
+    | primary  |
+    +----------+
+    ```
+
 2. On other TiDB clusters, execute `ADMIN SET BDR ROLE SECONDARY` to set them as the secondary clusters.
 3. Execute **replicable DDLs** on the primary cluster. The successfully executed DDLs will be replicated to the secondary clusters by TiCDC.
 

ticdc/ticdc-changefeed-config.md

@@ -74,6 +74,11 @@ case-sensitive = false
 # The format of this parameter is "h m s", for example, "1h30m30s".
 changefeed-error-stuck-duration = "30m"
 
+# The default value is false, indicating that bi-directional replication (BDR) mode is not enabled.
+# To set up BDR clusters using TiCDC, modify this parameter to `true` and set the TiDB clusters to BDR mode.
+# For more information, see https://docs.pingcap.com/tidb/stable/ticdc-bidirectional-replication.
+# bdr-mode = false
+
 [mounter]
 # The number of threads with which the mounter decodes KV data. The default value is 16.
 # worker-num = 16