private static void writePositional(PrintWriter pw, PositionalParamSpec positional,
            IParameterRenderer parameterRenderer, IParamLabelRenderer paramLabelRenderer) {
        pw.println();
        Text[][] rows = parameterRenderer.render(positional, paramLabelRenderer, COLOR_SCHEME);
        pw.printf("%s::%n", join(", ", rows[0][1], rows[0][3]));
        pw.printf("  %s%n", rows[0][4]);
        for (int i = 1; i < rows.length; i++) {
            pw.printf("+%n%s%n", rows[i][4]);
        }
    }
Returns the list of {@code ArgGroupSpec}s with a non-{@code null} heading.
docs/genadoc.java