doc: clarify sentence in event loop doc

Refs: https://github.com/nodejs/node/pull/8370#discussion-diff-77201863
PR-URL: https://github.com/nodejs/node/pull/8400


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>