go.net/ipv6: update package documentation
go.net/ipv6: update package documentation

- do word alignment; s/multicast group address/multicast group/

R=dave
CC=golang-dev
https://codereview.appspot.com/10685046

gigq/go.net,ssor/go.net,polydawn-ports/go.net,fernandez14/go.net,go-mirrors/go.net