doc: add `added` info for `dgram.setBroadcast()`

Since I was doing the necessary git spelunking anyway, I took the time
to add the YAML information into the docs about when `setBroadcast()`
first appeared in its current form.

PR-URL: https://github.com/nodejs/node/pull/6750


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>