doc: add `added:` info for dgram.*Membership()

Since I was doing the necessary git archaeology anyway, I took the time
to add YAML information to the docs about when `addMembership()` and
`dropMembership()` first appeared in their current forms.

PR-URL: https://github.com/nodejs/node/pull/6753
Ref: https://github.com/nodejs/node/issues/6578


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