Clarify _mm256_shuffle_epi8 documentation (#990)
Clarify _mm256_shuffle_epi8 documentation (#990)

The old documentation seemed to suggest that the low 4 bits of bytes in `b` would index into the 32-element array `a`. That's not quite right and you'd need 5 bits to index 32 elements.
rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch