storage: Document wiping formatted volume types
    
    When wiping a volume we just rewrite all the data of the volume, not
    only the content.  Since format gets overridden, we need to recreate the
    volume.  However we can't do that for every possible format out there.
    Since it was only coded for the ploop volume type, let's document what
    might be the consequences instead of forbidding it for every other
    format out there.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=868771
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>