doc: document that module.evaluate fulfills as undefined

Fixes: #37453
Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #37663
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

doc/api/vm.md

@@ -587,7 +587,7 @@ in the ECMAScript specification.
     [`Error`][]. Existing handlers for the event that have been attached via
     `process.on('SIGINT')` are disabled during script execution, but continue to
     work after that. **Default:** `false`.
-* Returns: {Promise}
+* Returns: {Promise} Fulfills with `undefined` upon success.
 
 Evaluate the module.