Br add experimental description for acceleration restore speed (#16266)

Author: Oreoxmt

br/br-snapshot-guide.md

@@ -71,7 +71,11 @@ tiup br restore full --pd "${PD_IP}:2379" \
 --storage "s3://backup-101/snapshot-202209081330?access-key=${access-key}&secret-access-key=${secret-access-key}"
 ```
 
-To address the potential bottleneck issue during restoration in large-scale Region scenarios, starting from v7.6.0, BR supports a coarse-grained Region scatter algorithm (experimental). You can enable this algorithm by specifying `--granularity="coarse-grained"`.
+> **Warning:**
+> 
+> The coarse-grained Region scatter algorithm (enabled by setting `--granularity="coarse-grained"`) is experimental. It is recommended that you use this feature to accelerate data recovery in clusters with up to 1,000 tables. Note that this feature does not support checkpoint restore.
+
+To further improve the restore speed of large clusters, starting from v7.6.0, BR supports a coarse-grained Region scatter algorithm (experimental) for faster parallel recovery. You can enable this algorithm by specifying `--granularity="coarse-grained"`. After it is enabled, BR can quickly split the restore task into a large number of small tasks and scatter them to all TiKV nodes in batches, thus fully utilizing the resources of each TiKV node for fast recovery in parallel.
 
 During restore, a progress bar is displayed in the terminal as shown below. When the progress bar advances to 100%, the restore task is completed and statistics such as total restore time, average restore speed, and total data size are displayed.