doc: tweak Go 1.4 release notes

LGTM=dave, dsymonds
R=golang-codereviews, dave, dsymonds
CC=golang-codereviews
https://codereview.appspot.com/184350043
go1.4

LGTM=bradfitz, minux, dsymonds
R=rsc, bradfitz, iant, dsymonds, minux
CC=golang-codereviews
https://codereview.appspot.com/188920043