doc/next: document cgo call overhead improvement

Change-Id: Ibac18513d2f76172665b4f3a4ea09b69ae0bdef3
Reviewed-on: https://go-review.googlesource.com/c/go/+/725664
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>