Gets the widgets whose on state is the field's value: none when it is Off; several for duplicated check boxes or radio
buttons in unison (which with Opt also includes widgets with the same export value).
public IReadOnlyList<PdfWidgetAnnotation> SelectedWidgets { get; }
ISO 32000-2 §12.7.5.2.3 (example 2) and §12.7.5.2.4, Table 229 (RadiosInUnison). Of radio buttons that share the on state without
RadiosInUnison, at most one is on: those whose AS shows it, when any does.