doc/cephfs: rearrange subvolume group information

Rearranging the documentation to align it with CephFS encapsulation design.

Signed-off-by: Indira Sawant <[email protected]>

Author: indirasawant

doc/cephfs/fs-volumes.rst

@@ -14,12 +14,12 @@ abstractions:
 
 * FS volumes, an abstraction for CephFS file systems
 
-* FS subvolumes, an abstraction for independent CephFS directory trees
-
 * FS subvolume groups, an abstraction for a directory level higher than FS
   subvolumes. Used to effect policies (e.g., :doc:`/cephfs/file-layouts`)
   across a set of subvolumes
 
+* FS subvolumes, an abstraction for independent CephFS directory trees
+
 Possible use-cases for the export abstractions:
 
 * FS subvolumes used as Manila shares or CSI volumes