docs: add missing comments to the Options API in the transition page (#…

…2721)

src/guide/built-ins/transition.md

@@ -407,6 +407,8 @@ export default {
 
     // called when the enter transition has finished.
     onAfterEnter(el) {},
+
+    // called when the enter transition is cancelled before completion.
     onEnterCancelled(el) {},
 
     // called before the leave hook.