document `--no-dns`
document `--no-dns`

Part of #1687.

paulbellamy/weave,paulbellamy/weave,weaveworks/weave,n054/weave,paulbellamy/weave,weaveworks/weave,n054/weave,n054/weave,paulbellamy/weave,n054/weave,weaveworks/weave,weaveworks/weave,n054/weave