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: #6750
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/dgram.md

@@ -283,6 +283,9 @@ not work because the packet will get silently dropped without informing the
 source that the data did not reach its intended recipient.
 
 ### socket.setBroadcast(flag)
+<!-- YAML
+added: v0.6.9
+-->
 
 * `flag` {Boolean}