doc: change "/s/..." links to be on golang.org

People viewing this locally will not have a /s/ on their local godoc.
tip.golang.org doesn't have one either.

Also change all golang.org links to https, to avoid mixed content
warnings when viewing https://golang.org/.

Fixes issue 9028.

LGTM=bradfitz, r
R=r, bradfitz
CC=adg, golang-codereviews
https://codereview.appspot.com/168250043
doc/go1.4.html: leave stack size at 2 kB

LGTM=r
R=khr, r
CC=golang-codereviews
https://codereview.appspot.com/165590043