[release-branch.go1.3] doc: document go1.3.1

««« CL 126060043 / e3cf4c202bd8
doc: document go1.3.1

LGTM=r
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/126060043
»»»

TBR=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/126070043