tidb-cloud: add incompatiblities between br and lightning (#15395)

tidb-cloud/backup-and-restore.md

@@ -15,6 +15,7 @@ This document describes how to back up and restore your TiDB Dedicated cluster d
 ## Limitations
 
 - Although TiDB Dedicated supports restoring user accounts from backups by default, it does not support restoring system variables stored in the `mysql` schema. 
+- It is recommended that you import data first, then perform a **manual** snapshot backup, and finally enable Point-in-time Restore. Because the data imported through the TiDB Cloud console **does not** generate change logs, it cannot be automatically detected and backed up. For more information, see [Import CSV Files from Amazon S3 or GCS into TiDB Cloud](/tidb-cloud/import-csv-files.md). 
 - If you turn on and off Point-in-time Restore multiple times, you can only choose a time point within the recoverable range after the most recent Point-in-time Restore is enabled. The earlier recoverable range is not accessible.
 - DO NOT modify the switches of **Point-in-time Restore** and **Dual Region Backup** at the same time.