docs(overlays): clarify how to remove an overlay (#28989)

In https://github.com/ionic-team/ionic-framework/issues/28981 there was
some confusion surrounding how to remove an overlay from the DOM if it
was never presented. The `dismiss` method will remove the overlay from
the DOM, but only if the overlay is visible. Otherwise, it's a no-op.

This PR updates the `dismiss` method docs for each overlay component to
note that developers can use the browser's remove method to remove the
element from the DOM.