better docstrings

you now see:

class_method(cls, *args, **kwargs) method of gi.types.GObjectMeta
instance
    make a black image
    usage:
       out = Vips.Image.black(width, height, bands = gint)

    where:
       out        - Output image, VipsImage

    required parameters:
       width      - Image width in pixels, gint
       height     - Image height in pixels, gint

    optional parameters:
       bands      - Number of bands in image, gint