doc: add return value to util.promisify

PR-URL: https://github.com/nodejs/node/pull/16040
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/util.md

@@ -461,6 +461,7 @@ added: v8.0.0
 -->
 
 * `original` {Function}
+* Returns: {Function}
 
 Takes a function following the common Node.js callback style, i.e. taking a
 `(err, value) => ...` callback as the last argument, and returns a version