feat(docs/metadata backup): add restore instructions (#4116)

docs/metadata_backup.md

@@ -1,6 +1,6 @@
 # Metadata backup
 
-> WARNING: Metadata backup is an experimental feature. Restore is not yet available and some unexpected issues may occur.
+> WARNING: Metadata backup is an experimental feature. Unexpected issues are possible, but unlikely.
 
 ## Introduction
 
@@ -11,21 +11,38 @@ In Xen Orchestra, Metadata backup is divided into two different options:
 * Pool metadata backup
 * XO configuration backup
 
-### How to use metadata backup
+### Performing a backup
 
-In the backup job section, when creating a new backup job, you will now have a choice between backing up VMs and backing up Metadata.
-![](https://user-images.githubusercontent.com/21563339/53413921-bd636f00-39cd-11e9-8a3c-d4f893135fa4.png)
+In the backup job section, when creating a new backup job, you will now have a choice between backing up VMs and backing up Metadata:
+![](./assets/metadata-1.png)
 
 When you select Metadata backup, you will have a new backup job screen, letting you choose between a pool metadata backup and an XO configuration backup (or both at the same time):
 
-![](https://user-images.githubusercontent.com/21563339/52416838-d2de2b00-2aea-11e9-8da0-340fcb2767db.png)
+![](./assets/metadata-2.png)
 
 Define the name and retention for the job.
 
-![](https://user-images.githubusercontent.com/21563339/52471527-65390a00-2b91-11e9-8019-600a4d9eeafb.png)
+![](./assets/metadata-3.png)
 
 Once created, the job is displayed with the other classic jobs.
 
-![](https://user-images.githubusercontent.com/21563339/52416802-c0fc8800-2aea-11e9-8ef0-b0c1bd0e48b8.png)
+![](./assets/metadata-4.png)
 
-> Restore for metadata backup jobs should be available in XO 5.33
+
+### Performing a restore
+
+> WARNING: restoring pool metadata completely overwrites the XAPI database of a host. Only perform a metadata restore if it is a new server with nothing running on it (eg replacing a host with new hardware).
+
+If you browse to the Backup NG Restore panel, you will now notice a Metadata filter button:
+
+![](./assets/metadata-5.png)
+
+If you click this button, it will show you Metadata backups available for restore:
+
+![](./assets/metadata-6.png)
+
+You can see both our Xen Orchestra config backup, and our pool metadata backup. To restore one, simply click the blue restore arrow, choose a backup date to restore, and click OK:
+
+![](./assets/metadata-7.png)
+
+That's it!