[release-branch.go1.12] doc: document Go 1.12.17
Change-Id: I95f81f269e742ac058cb3e6404cc43beb2428926
Reviewed-on: https://go-review.googlesource.com/c/go/+/219200
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>