ticdc: update ticdc failed changefeed gc mechanism description (#13843)

ticdc/ticdc-changefeed-overview.md

@@ -19,7 +19,7 @@ The states in the preceding state transfer diagram are described as follows:
 - `Stopped`: The replication task is stopped, because the user manually pauses the changefeed. The changefeed in this state blocks GC operations.
 - `Error`: The replication task returns an error. The replication cannot continue due to some recoverable errors. The changefeed in this state keeps trying to resume until the state transfers to `Normal`. The changefeed in this state blocks GC operations.
 - `Finished`: The replication task is finished and has reached the preset `TargetTs`. The changefeed in this state does not block GC operations.
-- `Failed`: The replication task fails. Due to some unrecoverable errors, the replication task cannot resume and cannot be recovered. The changefeed in this state does not block GC operations.
+- `Failed`: The replication task fails. Due to some unrecoverable errors, the replication task cannot resume and cannot be recovered. TiCDC retains the data associated with the failed changefeed for 24 hours, preventing it from being garbage collected.
 
 The numbers in the preceding state transfer diagram are described as follows.