[css-view-transitions-2] Clarify what happens if a transition is initiated during a rendering update

Currently it's defined that the new state is captured on the next frame. But what happens if we're already in the middle of a frame? e.g.
```html
<head>
   <script>
     requestAnimationFrame(() => {
       startViewTranstiion(...);
     });
   </script>
</head>
```