v8.5.6: Add docs about the cluster_id row in mysql.tidb table (#20280) (#22566)

Author: ti-chi-bot

mysql-schema/mysql-schema.md

@@ -33,6 +33,7 @@ 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` (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