simd/archsimd: reword documentation for some operations
- Min/Max: make it clear it is elementwise.
- RoundToEven: clarify it is rounding tie to even.
- MulEvenWiden: use mathematical form of the index.
- CopySign: use parameter names directly.
- ConcatShiftBytesRight: rename the parameter.
Change-Id: I4cf0773c4daf3e3bf7b26e79d84ac5c2a9145c88
Reviewed-on: https://go-review.googlesource.com/c/go/+/731421 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>