document `weave launch-router --trusted-subnets`
document `weave launch-router --trusted-subnets`

and eliminate duplication

Fixes #1842.

tomwilkie/weave,weaveworks/weave,rade/weave,dpw/weave,paulbellamy/weave,weaveworks/weave,tomwilkie/weave,rade/weave,n054/weave,paulbellamy/weave,tomwilkie/weave,tomwilkie/weave,dpw/weave,rade/weave,dpw/weave,n054/weave,tomwilkie/weave,rade/weave,paulbellamy/weave,n054/weave,dpw/weave,rade/weave,weaveworks/weave,n054/weave,weaveworks/weave,dpw/weave,n054/weave,paulbellamy/weave