There's no standard way to do this. There's optional which needs to be kept unchanged (because it's so widely used, not necessarily because it's optimal), and that intersects with keyword-only. The description for d here is a little ugly, but I don't have a better suggestion:

def my_func(a, *, b, c, d=None):
    """ My function does stuff

    a : object
        Something about `a`.
    b : object, keyword-only
        Something about `b`.
    c : object, keyword-only
        Something about `c`.
    d : object, optional, keyword-only
    """

Personal opinion: keyword-only (and positional-only) are just syntactic usage aspects. They don't carry information for understanding the function and its parameters. I therefore propose that it's sufficient to let the user infer the usage from the signature and would not document that aspect in the parameters section at all.