Adjust relationship between rx_fold behaviour, documentation and reality

`rx_fold()` was processing arguments in the wrong order. In particular, the canonical example:

    #define add(x, y) y + x
    int sum = rx_fold(add, 0, 1, 2, 3, 4, 5);

actually expanded to:

    int sum = 1 + 2 + 3 + 4 + 5 + 0;

Now it expands to `0 + 1 + 2 + 3 + 4 + 5` (and documentation no longer claims it magically adds `+ 6`).