scrypt comment for p>1

[Solar Designer <solar@openwall.com>]

Colin,

You might want to add a source code comment into crypto_scrypt-*.c:
crypto_scrypt() saying that if the "for (i = 0; i < p; i++)" loop is
actually parallelized, then separate instances of XY and V will need to
be allocated for each thread (and you might want to emphasize in the
comment that there may be fewer than "p" of these).

Thanks,

Alexander