doc: flesh out tools section in Go 1.9 release notes

doc: flesh out tools section in Go 1.9 release notes

I thought I was almost done, but had forgot the tools section, hidden
in comments.

Move the comments to a <pre> block, so it's visible in the HTML.

Updates #20587

Change-Id: I1dc22c63d9ee297e44bbb742f03b4a722247dbe8
Reviewed-on: https://go-review.googlesource.com/45811
Reviewed-by: Ian Lance Taylor <iant@golang.org>