doc/source: Add documentation for added features

Signed-off-by: hamistao <pedro.ribeiro@canonical.com>

doc/source/storage-pools.rst

@@ -137,10 +137,48 @@ following methods are available:
     changes to the LXD server.
   - `delete` - delete a storage volume object.  Note that the object is,
     therefore, stale after this action.
+  - `restore` - Restore the volume from a snapshot using the snapshot name.
 
 .. note:: `raw_put` and `raw_patch` are availble (but not documented) to allow
         putting and patching without syncing the object back.
 
+Storage Volume Snapshots
+------------------------
+
+Storage Volume Snapshots are represented as `StorageVolumeSnapshot` objects and
+stored in `StorageVolume` objects and represent snapshots of custom storage volumes.
+On the `pylxd` API they are accessed from a storage volume object that, in turn,
+is accessed from a storage pool object:
+
+.. code:: Python
+
+    storage_pool = client.storage_pools.get('pool1')
+    volumes = storage_pool.volumes.all()
+    custom_volume = storage_pool.volumes.get('custom', 'vol1')
+    volumes = custom_volume.snapshots.all()
+    a_snapshot = custom_volume.snapshots.get('snap0')
+
+Methods available on `<storage_volume_object>.snapshots`
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The following methods are accessed from the `snapshots` attribute on the `StorageVolume` object:
+
+  - `all` - Get all the snapshots of the storage volume.
+  - `get` - Get a single snapshot using its name.
+  - `create` - Take a snapshot on the current stage of the storage volume. The new snapshot's
+  name and expiration date can be set, default name is in the format "snapX".
+  - `exists` - Return a boolean for whether a storage volume snapshot exists, uses the snapshot name.
+
+Methods available on the storage snapshot object
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Once in possession of a storage volume snapshot object from the `pylxd` API, the
+following methods are available:
+
+  - `restore` - Restore the volume from the snapshot.
+  - `delete` - Delete a previously taken snapshot using its name.
+  - `is_expired` - Return true if the snapshot is expired, false otherwise.
+
 .. links
 
 .. _LXD Storage Pools: https://documentation.ubuntu.com/lxd/en/latest/storage/