TiCDC: Update ticdc-overview.md (#2694)

* Update ticdc-overview.md

* update wording

* Update ticdc/ticdc-overview.md

* Update ticdc/ticdc-overview.md

Co-authored-by: amyangfei <amyangfei@gmail.com>

Co-authored-by: Ran <huangran@pingcap.com>
Co-authored-by: amyangfei <amyangfei@gmail.com>
Co-authored-by: pingcap-github-bot <sre-bot@pingcap.com>

Author: 4 people

ticdc/ticdc-overview.md

@@ -7,6 +7,10 @@ aliases: ['/docs/dev/reference/tools/ticdc/overview/']
 
 # TiCDC Overview
 
+> **Note:**
+> 
+> TiCDC is experimental. It is **not recommended** to use this feature in the production environment.
+
 [TiCDC](https://github.com/pingcap/ticdc) is a tool for replicating the incremental data of TiDB. This tool is implemented by pulling TiKV change logs. It can restore data to a consistent state with any upstream TSO, and provides [TiCDC Open Protocol](/ticdc/ticdc-open-protocol.md) to support other systems to subscribe to data changes.
 
 ## TiCDC Architecture
@@ -41,13 +45,28 @@ Currently, the TiCDC sink component supports replicating data to the following d
 - Databases compatible with MySQL protocol. The sink component provides the final consistency support.
 - Kafka based on the TiCDC Open Protocol. The sink component ensures the row-level order, final consistency or strict transactional consistency.
 
-### Black and white table lists
-
-You can write blacklist and whitelist filtering rules to filter or only replicate all changed data in certain databases or tables. The filtering rules are similar to those of MySQL such as `replication-rules-db` or `replication-rules-table`.
-
 ## Restrictions
 
 To replicate data to TiDB or MySQL, you must ensure that the following requirements are satisfied to guarantee data correctness:
 
 - The table to be replicated has the primary key or a unique index.
 - If the table to be replicated only has unique indexes, each column of at least one unique index is explicitly defined in the table schema as `NOT NULL`.
+
+### Unsupported scenarios
+
+Currently, The following scenarios are not supported:
+
+- The TiKV cluster that uses RawKV alone.
+- The [new framework for collations](/character-set-and-collation.md#new-framework-for-collations) in TiDB v4.0. Before you enable this feature, make sure that the downstream cluster is the TiDB cluster with the same collation as those in the upstream; otherwise, inconsistent collations lead to the issue that the data cannot be located.
+- The [DDL operation `CREATE SEQUENCE`](/sql-statements/sql-statement-create-sequence.md) and the [SEQUENCE function](/sql-statements/sql-statement-create-sequence.md#sequence-function) in TiDB v4.0. When the upstream TiDB uses `SEQUENCE`, TiCDC ignores `SEQUENCE` DDL operations/functions performed upstream. However, DML operations using `SEQUENCE` functions can be correctly replicated.
+- The [TiKV Hibernate Region](https://github.com/tikv/tikv/blob/master/docs/reference/configuration/raftstore-config.md#hibernate-region). TiCDC prevents the Region from entering the hibernated state.
+- The scheduling of existing replication tables to new TiCDC nodes, after the capacity of the TiCDC cluster is scaled out.
+- The replication of Black and white table lists.
+
+## Manage TiCDC Cluster and Replication Tasks
+
+For details, see [Manage TiCDC Cluster and Replication Tasks](/ticdc/manage-ticdc.md).
+
+## TiCDC Open Protocol
+
+TiCDC Open Protocol is a row-level data change notification protocol that provides data sources for monitoring, caching, full-text indexing, analysis engines, and master-slave replication between different databases. TiCDC complies with TiCDC Open Protocol and replicates data changes of TiDB to third-party data medium such as MQ (Message Queue). For more information, see [TiCDC Open Protocol](/ticdc/ticdc-open-protocol.md).