Documentation: siphash: disambiguate HalfSipHash algorithm from hsiphash functions

Fix the documentation for the hsiphash functions to avoid conflating the
HalfSipHash algorithm with the hsiphash functions, since these functions
actually implement either HalfSipHash or SipHash, and random.c now uses
HalfSipHash (in a very special way) without the hsiphash functions.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>