cherry pick pingcap#3539 to release-4.0

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

br/backup-and-restore-tool.md

@@ -14,6 +14,12 @@ aliases: ['/docs/stable/br/backup-and-restore-tool/','/docs/v4.0/br/backup-and-r
 - Currently, TiDB does not support backing up and restoring partitioned tables.
 - Currently, you can perform restoration only on new clusters.
 - It is recommended that you execute multiple backup operations serially. Otherwise, different backup operations might interfere with each other.
+- BR supports operations only between clusters with the same [`new_collations_enabled_on_first_bootstrap`](/character-set-and-collation.md#collation-support-framework) value because BR only backs up KV data. If the cluster to be backed up and the cluster to be restored use different collations, the data validation fails. Therefore, before restoring a cluster, make sure that the switch value from the query result of the `select VARIABLE_VALUE from mysql.tidb where VARIABLE_NAME='new_collation_enabled';` statement is consistent with that during the backup process.
+
+    - For v3.1 clusters, the new collation framework is not supported, so you can see it as disabled.
+    - For v4.0 clusters, check whether the new collation is enabled by executing `SELECT VARIABLE_VALUE FROM mysql.tidb WHERE VARIABLE_NAME='new_collation_enabled';`.
+
+    For example, assume that data is backed up from a v3.1 cluster and will be restored to a v4.0 cluster. The `new_collation_enabled` value of the v4.0 cluster is `true`, which means that the new collation is enabled in the cluster to be restored when this cluster is created. If you perform the restore in this situation, an error might occur.
 
 ## Recommended deployment configuration