net: document ReadMsgUnix, WriteMsgUnix
net: document ReadMsgUnix, WriteMsgUnix

Fixes issue 3247.

R=golang-dev, jsing
CC=golang-dev
http://codereview.appspot.com/5784051
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go