doc: ISO volumes can be created from copies

Signed-off-by: hamistao <[email protected]>

doc/explanation/storage.md

@@ -161,7 +161,7 @@ Each storage volume uses one of the following content types:
 
 `iso`
 : This content type is used for custom ISO volumes.
-  A custom storage volume of type `iso` can only be created by importing an ISO file using [`lxc storage volume import`](lxc_storage_volume_import.md).
+  A custom storage volume of type `iso` can only be created by importing an ISO file using [`lxc storage volume import`](lxc_storage_volume_import.md) or by copying another volume.
 
   Custom storage volumes of content type `iso` can only be attached to virtual machines.
   They can be attached to multiple machines simultaneously as they are always read-only.