add wait async ddl description (#18900) (#19148)

ticdc/ticdc-ddl.md

@@ -50,6 +50,8 @@ The allow list of DDL statements supported by TiCDC is as follows:
 
 When the downstream is TiDB, TiCDC executes `ADD INDEX` and `CREATE INDEX` DDL operations asynchronously to minimize the impact on changefeed replication latency. This means that, after replicating `ADD INDEX` and `CREATE INDEX` DDLs to the downstream TiDB for execution, TiCDC returns immediately without waiting for the completion of the DDL execution. This avoids blocking subsequent DML executions.
 
+During the execution of the `ADD INDEX` or `CREATE INDEX` DDL operation in the downstream, when TiCDC executes the next DDL operation of the same table, this DDL operation might be blocked in the `queueing` state for a long time. This can cause TiCDC to repeatedly execute this DDL operation, and if retries take too long, it might lead to replication task failure. Starting from v7.5.4, if TiCDC has the `SUPER` permission of the downstream database, it periodically runs `ADMIN SHOW DDL JOBS` to check the status of asynchronously executed DDL tasks. TiCDC will wait for index creation to complete before proceeding with replication. Although this might increase replication latency, it avoids replication task failure.
+
 > **Note:**
 >
 > - If the execution of certain downstream DMLs relies on indexes that have not completed replication, these DMLs might be executed slowly, thereby affecting TiCDC replication latency.

ticdc/ticdc-split-update-behavior.md

@@ -113,7 +113,7 @@ Therefore, TiCDC splits these two events into four events, that is, deleting rec
 Starting from v6.5.10, v7.1.6, and v7.5.3, when using a non-MySQL sink, TiCDC supports controlling whether to split primary or unique key `UPDATE` events via the `output-raw-change-event` parameter, as described in the GitHub issue [#11211]( https://github.com/pingcap/tiflow/issues/11211). The specific behavior of this parameter is as follows:
 
 - When you set `output-raw-change-event = false`, if the primary key or non-null unique index value is modified in an `UPDATE` event, TiCDC splits the event into `DELETE` and `INSERT` events and ensures that all events follow the sequence of `DELETE` events preceding `INSERT` events.
-- When you set `output-raw-change-event = true`, TiCDC does not split `UPDATE` events. Note that when the primary key of a table is a clustered index, updates to the primary key are still split into `DELETE` and `INSERT` events in TiDB, and such behavior is not affected by the `output-raw-change-event` parameter.
+- When you set `output-raw-change-event = true`, TiCDC does not split `UPDATE` events, and the consumer side is responsible for dealing with the problems described in [Split primary or unique key `UPDATE` events for non-MySQL sinks](/ticdc/ticdc-split-update-behavior.md#split-primary-or-unique-key-update-events-for-non-mysql-sinks). Otherwise there might be a risk of data inconsistency. Note that when the primary key of a table is a clustered index, updates to the primary key are still split into `DELETE` and `INSERT` events in TiDB, and such behavior is not affected by the `output-raw-change-event` parameter.
 
 > **Note**
 >