add_dim_coord Documentation is misleading

[pelson]

The docs for
cube.add_dim_coord(dim_coord, data_dim=None):
state the following:

    Add a CF coordinate to the cube.

    Args:

        dim_coord

            The iris.coords.DimCoord instance to add to the cube.

    Kwargs:

        data_dim

            Integer giving the data dimension spanned by the coordinate.

    Raises a ValueError if a coordinate with identical metadata already exists on the cube or if a coord already exists for the given dimension.

    See also Cube.remove_coord().

However, data_dim is not an optional kwarg.

Please update the documentation for this method.