doc: update promisify() docs with behavior when bad arguments are passed

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.

Fixes: https://github.com/nodejs/node/issues/17569

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc