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