Add missing "Volume version 2 only" message in backup.rst

Former patchs add some volume v2 only options for backup but
miss "Volume version 2 only" message. This patch adds them.

Change-Id: Id50db25617776b6ee0d0ad6b4b82bd443b940343

Author: Huanxuan Ao

doc/source/command-objects/backup.rst

@@ -16,6 +16,7 @@ Create new backup
         [--container <container>]
         [--name <name>]
         [--description <description>]
+        [--snapshot <snapshot>]
         [--force]
         <volume>
 
@@ -35,10 +36,14 @@ Create new backup
 
     Snapshot to backup (name or ID)
 
+    *Volume version 2 only*
+
 .. option:: --force
 
     Allow to back up an in-use volume
 
+    *Volume version 2 only*
+
 .. _backup_create-backup:
 .. describe:: <volume>