doc: amend installation instructions

Fixes issue 8674.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/139320044
runtime: mark freedefer and deferclass go:nosplit

This should make deferreturn nosplit all the way down,
which should fix the current windows/amd64 failure.
If not, I will change StackCopyAlways back to 0.

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/135600043