simd/archsimd: correct documentation for pairwise operations

simd/archsimd: correct documentation for pairwise operations

For Add/SubPairs(Saturated?), the documented result element order
is wrong. Corrected.

Also, for 256-bit vectors, this is a grouped operation. So name it
with the Grouped suffix to be clear.

Change-Id: Idfd0975cb4a332b2e28c898613861205d26f75b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/732020
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>