go.net/ipv6: update comment on multicast test
go.net/ipv6: update comment on multicast test

That's the reason why we now skip the test on freebsd.

R=dave
CC=golang-dev
https://golang.org/cl/10781043

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