cdc: add warn for event filter (pingcap#19223)

ticdc/ticdc-changefeed-config.md

@@ -105,7 +105,7 @@ rules = ['*.*', '!test.*']
 
 # The second event filter rule.
 # matcher = ["test.fruit"] # matcher is an allow list, which means this rule only applies to the fruit table in the test database.
-# ignore-event = ["drop table", "delete"] # Ignore the `drop table` DDL events and the `delete` DML events.
+# ignore-event = ["drop table", "delete"] # Ignore the `drop table` DDL events and the `delete` DML events. Note that when a value in the clustered index column is updated in TiDB, TiCDC splits an `UPDATE` event into `DELETE` and `INSERT` events. TiCDC cannot identify such events as `UPDATE` events and thus cannot correctly filter out such events.
 # ignore-sql = ["^drop table", "alter table"] # Ignore DDL statements that start with `drop table` or contain `alter table`.
 # ignore-insert-value-expr = "price > 1000 and origin = 'no where'" # Ignore insert DMLs that contain the conditions "price > 1000" and "origin = 'no where'".