doc: fix event.cancelBubble documentation

Setter of `event.cancelBubble` is a no-op if the value passed to it is
not `true`.

Refs: https://dom.spec.whatwg.org/#interface-event
PR-URL: #45986
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/events.md

@@ -1748,14 +1748,16 @@ added: v14.5.0
 
 This is not used in Node.js and is provided purely for completeness.
 
-#### `event.cancelBubble()`
+#### `event.cancelBubble`
 
 <!-- YAML
 added: v14.5.0
 -->
 
-Alias for `event.stopPropagation()`. This is not used in Node.js and is
-provided purely for completeness.
+* Type: {boolean}
+
+Alias for `event.stopPropagation()` if set to `true`. This is not used
+in Node.js and is provided purely for completeness.
 
 #### `event.cancelable`