doc: more go1.8.html content

TBR=See https://golang.org/cl/33244 and review there.

Updates #17929

Change-Id: I37b49318a9203b16c0c788926039288b99a36ce5
Reviewed-on: https://go-review.googlesource.com/33450
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz