**API**: Tweak class docstrings and inheritance a bit
    
    This unfortunately breaks the API, but is worth it for future
    consistency. Result subclasses should specify only their own
    args, never explicitly specify args from parent classes; these
    should only be taken as `*args` and `**kwargs` and passed
    through. `FedoraImageResult` was taking `cid` from its parent
    class (`ComposeResult`), so stop it doing that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>