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: #17593
Fixes: #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>

doc/api/util.md

@@ -499,8 +499,10 @@ 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.js style callback, it will still be passed a Node.js style callback
+as its last argument.
 
 ### Custom promisified functions
 
@@ -535,6 +537,8 @@ doSomething[util.promisify.custom] = (foo) => {
   });
 };
 ```
+If `promisify.custom` is defined but is not a function, `promisify()` will
+throw an error.
 
 ### util.promisify.custom
 <!-- YAML