br: add compatibility between log backup and PITR (#20485)

Author: YuJuncen

br/br-pitr-manual.md

@@ -495,3 +495,25 @@ tiup br restore point --pd="${PD_IP}:2379"
 --master-key-crypter-method aes128-ctr
 --master-key "local:///path/to/master.key"
 ```
+
+### Compatibility between ongoing log backup and snapshot restore
+
+Starting from v9.0.0, when a log backup task is running, if all of the following conditions are met, you can still perform snapshot restore (`br restore [full|database|table]`) and allow the restored data to be properly recorded by the ongoing log backup (hereinafter referred to as "log backup"):
+
+- The node performing backup and restore operations has the following necessary permissions: 
+    - Read access to the external storage containing the backup source, for snapshot restore
+    - Write access to the target external storage used by the log backup
+- The target external storage for the log backup is Amazon S3 (`s3://`), Google Cloud Storage (`gcs://`), or Azure Blob Storage (`azblob://`).
+- The data to be restored uses the same type of external storage as the target storage for the log backup.
+- Neither the data to be restored nor the log backup has enabled local encryption. For details, see [log backup encryption](#encrypt-the-log-backup-data) and [snapshot backup encryption](/br/br-snapshot-manual.md#encrypt-the-backup-data).
+
+If any of the above conditions are not met, or if you need to perform a point-in-time recovery, while a log backup task is running, BR refuses to proceed with the data recovery. In this case, you can complete the recovery by following these steps:
+
+1. [Stop the log backup task](#stop-a-log-backup-task).
+2. Perform the data restore.
+3. After the restore is complete, perform a new snapshot backup.
+4. [Restart the log backup task](#restart-a-log-backup-task).
+
+> **Note:**
+>
+> When restoring a log backup that contains records of snapshot (full) restore data, you must use BR v9.0.0 or later. Otherwise, restoring the recorded full restore data might fail.