go/build: update docs for GOOS.go change

Forgotten in https://codereview.appspot.com/147690043/

Update Issue 8838

LGTM=r
R=r
CC=golang-codereviews, rsc
https://codereview.appspot.com/152220045
strings: use fast path for IndexRune

Noticed while reviewing https://codereview.appspot.com/147690043/

I'd never seen anybody use IndexRune before, and
unsurprisingly it doesn't use the other fast paths in the
strings/bytes packages. IndexByte uses assembly.

Also, less code this way.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/147700043