docs: transition hook add comment (#2441)

* docs: transition hook add comment

* Update src/guide/built-ins/transition.md

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

---------

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

Author: btea

src/guide/built-ins/transition.md

@@ -362,6 +362,8 @@ function onEnter(el, done) {
 
 // called when the enter transition has finished.
 function onAfterEnter(el) {}
+
+// called when the enter transition is cancelled before completion.
 function onEnterCancelled(el) {}
 
 // called before the leave hook.