v8.5.6 release notes: add info about cluster_id (#22766)

Author: qiancai

mysql-schema/mysql-schema.md

@@ -34,7 +34,7 @@ These system tables contain grant information about user accounts and their priv
     * `tidb_server_version`: the version information of TiDB when it is initialized. Note that this value is read-only and cannot be modified.
     * `system_tz`: the system time zone of TiDB.
     * `new_collation_enabled`: whether TiDB has enabled the [new framework for collations](/character-set-and-collation.md#new-framework-for-collations). Note that this value is read-only and cannot be modified.
-    * `cluster_id`: the unique identifier of the TiDB cluster. Note that this value is read-only and cannot be modified.
+    * `cluster_id` (New in v8.5.6): the unique identifier of the TiDB cluster. Note that this value is read-only and cannot be modified.
 
 ## Server-side help system tables
 

releases/release-8.5.6.md

@@ -120,6 +120,12 @@ For TiDB clusters newly deployed in v8.5.5 (that is, not upgraded from versions
 | TiKV | [`resource-metering.enable-network-io-collection`](https://docs.pingcap.com/tidb/v8.5/tikv-configuration-file#enable-network-io-collection-new-in-v856) | Newly added | Controls whether TiKV network traffic and logical I/O metrics are additionally collected in Top SQL. The default value is `false`. |
 | TiCDC | [`sink.csv.output-field-header`](https://docs.pingcap.com/tidb/v8.5/ticdc-csv#use-csv) | Newly added | Controls whether a header row is output in CSV files. The default value is `false`. This parameter applies only to the TiCDC new architecture. |
 
+### System table changes
+
+| System table | Change type | Description |
+| -------- | -------- | -------- |
+| [`mysql.tidb`](https://docs.pingcap.com/tidb/v8.5/mysql-schema#cluster-status-system-tables) | Modified | Adds the `cluster_id` field, which represents the unique identifier of a TiDB cluster. Note that `cluster_id` is read-only and cannot be modified. |
+
 ## Deprecated features
 
 - Starting from v8.5.6, statistics Version 1 (`tidb_analyze_version = 1`) is deprecated and will be removed in a future release. It is recommended that you use statistics Version 2 (`tidb_analyze_version = 2`) and [migrate existing objects that use statistics Version 1 to Version 2](https://docs.pingcap.com/tidb/v8.5/statistics#switch-between-statistics-versions) for more accurate statistics.