[release-branch.r57] docs: Update notes for 3-day Go course.

««« CL 4605041 / 71776ebc7416
docs: Update notes for 3-day Go course.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4605041
»»»

R=adg
CC=golang-dev
https://golang.org/cl/4635041