doc: clearify promisify behavior for bad arguments

Currently the documentation states that promisify() will result in
undefined behavior if bad arguments are passed. This is not
necessarily the case, since the behavior is well defined, but just
not useful.

PR-URL: https://github.com/nodejs/node/pull/17593
Fixes: https://github.com/nodejs/node/issues/17569


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>