def comment_radii(r_inner, r_outer, r):
    """
    Convert radii defining the cluster and sub-spheres to a human-readable string.

    Args:
        r_inner (float): Inner sub-sphere radius.
        r_outer (float): Outer sub-sphere radius.
        r (float): Radius of the entire cluster.

    Returns:
        (*str*) -- A comment describing the cluster radii.
    """
    return 'inner, outer, and cluster radii = ' + ' '.join(str(rad) for rad in [r_inner, r_outer, r]) + '; '