feat(backup): add doc for cluster backups

docs/cloud/backups.md

@@ -21,6 +21,8 @@ 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 Swarm (multi-shard) data stores see [cluster backup limitations](#swarm-multi-shard-backups).
+
 ## Manual On-Demand Backups
 
 To create a manual on-demand backup, follow the steps below:
@@ -63,6 +65,25 @@ 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**.
 
+## Swarm (multi-shard) Backups
+
+Dragonfly Cloud supports backups for Swarm (multi-shard) data stores. A multi shard datastore 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 is enforced by the backup**: Data is partitioned across shards by hash slot ranges.
+  When restoring, the slot distribution of the target data store is overwritten to match the shard
+  distribution recorded in the backup. The target data store will reflect the original slot layout
+  after the restore completes.
+- **Swarm backups cannot be restored to a single-shard data store**: A backup taken from a
+  multi-shard swarm data store cannot be restored to a single-shard (non-clustered) data store.
+  The reverse is supported — a single-shard backup can be restored to a swarm data store.
+
 ## Viewing and Deleting Backups
 
 - You can view all backups under the [Account > Backups](https://dragonflydb.cloud/account/backups) tab.