crypto/internal/nistec: port cleanups and docs from p256_asm.go to purego

crypto/internal/nistec: port cleanups and docs from p256_asm.go to purego

Change-Id: Ieaad0692f4301cc301a0dd2eadca2f2f9e96bff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/627942
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>