IonInput missing reference to IonItem CSS variables

Looking at the [IonInput](https://ionicframework.com/docs/api/input) component styling I opened an [useless issues](https://github.com/ionic-team/ionic/issues/17498) because I was unable to find a way to remove padding from the right side.

I think that [IonInput Page](https://ionicframework.com/docs/api/input) should add reference that, as inner element of IonItem, those are the CSS variables that has to be used.