doc: clarify collaborators & ctc members relationships

inspired by this conversation
https://github.com/nodejs/node/pull/7183#discussion_r73786541

PR-URL: https://github.com/nodejs/node/pull/7996


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>