[release-branch.go1.9] doc: document Go 1.8.6

Update golang/go#23515

Change-Id: Id334d8663bf4cbb68f224d1bba4c9ad3855f8aae
Reviewed-on: https://go-review.googlesource.com/89155


Reviewed-by: Andrew Gerrand <adg@golang.org>
(cherry picked from commit 6c27114c)
Reviewed-on: https://go-review.googlesource.com/89176


Reviewed-by: Andrew Bonventre <andybons@golang.org>