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://golang.org/cl/10685046

cmars/tools,subuk/net,zabawaba99/net,muzining/net,flowlo/go-net,dafyddcrosby/net,mckinney-subgraph/net,foreversmart/net,veliamsli/net,golang/net,dafyddcrosby/net,yangxianjun/net,mckinney-subgraph/net,cmars/tools,chenzlabs/net,zabawaba99/net,albertjin/golang-net,DramaFever/golang.org-x-net,gomini/net,cesanta/goxnet,DramaFever/golang.org-x-net,flowlo/go-net,cesanta/goxnet,kiip/net,w4ngyi/net,gomini/net,foreversmart/net,raboof/net,subuk/net,kiip/net,chenzlabs/net,cmars/tools,raboof/net,muzining/net,veliamsli/net,albertjin/golang-net,w4ngyi/net,cmars/tools,yangxianjun/net