feat(backup): add doc for cluster backups

Signed-off-by: Abhradeep Chakraborty <abhradeep@dragonflydb.io>

Author: Abhra303

docs/cloud/backups.md

@@ -21,6 +21,9 @@ from a remote storage system is also supported.
 Existing backups in your Dragonfly Cloud account can be [restored to an active data store](#restoring-from-backup)
 at any time.
 
+For clustered (multi-shard) data stores, please review the [cluster backup limitations](#cluster-datastore-backups)
+before restoring.
+
 ## Manual On-Demand Backups
 
 To create a manual on-demand backup, follow the steps below:
@@ -63,6 +66,25 @@ To restore a backup in an existing data store, follow the steps below, **with ca
 - Make **100% sure you select the correct backup** from the dropdown.
 - Click **Restore**.
 
+## Cluster Datastore Backups
+
+Dragonfly Cloud supports backups for clustered (multi-shard) data stores. A cluster backup captures
+the full dataset distributed across all shards at the point in time the backup was taken.
+
+### Limitations
+
+- **Shard count must match on restore**: The target data store must have the same number of shards
+  as the data store at the time the backup was created. Restoring a cluster backup to a data store
+  with a different shard count is not supported.
+- **Shard distribution must be identical**: Data is partitioned across shards by hash slot ranges.
+  Because each shard's backup encodes data bound to a specific slot range, the slot layout of the
+  target data store must exactly match the original. A mismatch will result in a restore failure.
+- **Cross-tier restore is not supported**: A backup taken from a clustered data store cannot be
+  restored to a single-shard (non-clustered) data store, and vice versa.
+
+***CAUTION: Before initiating a restore, verify that the target data store's shard count and
+configuration match the data store that the backup was originally taken from.***
+
 ## Viewing and Deleting Backups
 
 - You can view all backups under the [Account > Backups](https://dragonflydb.cloud/account/backups) tab.