docs: recommend setting e.returnValue (#33645)

Co-authored-by: Cheng Zhao <zcbenz@gmail.com>

docs/api/browser-window.md

@@ -464,7 +464,7 @@ window.onbeforeunload = (e) => {
   // a non-void value will silently cancel the close.
   // It is recommended to use the dialog API to let the user confirm closing the
   // application.
-  e.returnValue = false // equivalent to `return false` but not recommended
+  e.returnValue = false
 }
 ```