doc: missing argument types for events methods

Refs: https://github.com/nodejs/node/issues/9399
PR-URL: https://github.com/nodejs/node/pull/11802


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>