net: fix documentation for UnixAddr
net: fix documentation for UnixAddr

Also simplifies ResolveUnixAddr.

R=golang-dev, dave, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7510047

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang