doc: argument types for assert methods

Refs: https://github.com/nodejs/node/issues/9399

PR-URL: https://github.com/nodejs/node/pull/11548


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>