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: nodejs#17569 (comment)

doc/api/util.md

@@ -552,8 +552,10 @@ If there is an `original[util.promisify.custom]` property present, `promisify`
 will return its value, see [Custom promisified functions][].
 
 `promisify()` assumes that `original` is a function taking a callback as its
-final argument in all cases, and the returned function will result in undefined
-behavior if it does not.
+final argument in all cases. If `original` is not a function, `promisify()` 
+will throw an error. If `original` is a function but its last argument is not a 
+node-style callback, it will be still be passed a node-style callback as its last
+argument anyways.
 
 ### Custom promisified functions
 
@@ -588,6 +590,8 @@ doSomething[util.promisify.custom] = (foo) => {
   });
 };
 ```
+If `promisify.custom` is defined but is not a function, `promisify()` will 
+throw an error.
 
 ### util.promisify.custom
 <!-- YAML