Update async-suspense.md

Author: tobi-fis

docs/guide/advanced/async-suspense.md

@@ -140,6 +140,7 @@ test('Async component', async () => {
   // ...
 })
 ```
+Note: To access your `Async` components' underlying `vm` instance, use the return value of `wrapper.findComponent(Async)`. Since a new component is defined and mounted in this scenario, the wrapper returned by `mount(TestComponent)` contains its' own (empty) `vm`.
 
 ## Conclusion