Note about stacking modals is not complete

In the docs of the ion-modal (https://ionicframework.com/docs/api/modal), I found a note about adding CSS in order to darken the background of stacking modals:

```
ion-modal.stack-modal {
--box-shadow: 0 28px 48px rgba(0, 0, 0, 0.4);
--backdrop-opacity: var(--ion-backdrop-opacity, 0.32);
}
```

I noticed that this CSS gets overwritten by styles of Ionic. It was necessary to add `!important` to the custom styles above in order to make a backdrop getting darker with multiple modals.