simd/archsimd: reword documentation for conversion ops
Use more compact wording for extension, truncation, and
saturation. Say that we pack the results to the low elements and
zero the high elements if and only if the result has more elements.
Change-Id: Iae98d3c6ea6b5b5fa0acd548471e8d6c70a26d2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/730940
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>