Initializes a new instance of the PdfRadioButton class.
public PdfRadioButton(PdfPage page, PdfRectangle rectangle, string state)
The page the widget is on: a page of the document the field is created in.
The widget's rectangle, in default user space.
The button's on state, written as a name (its export value); not Off.
ISO 32000-2 §12.7.5.2.4 and §12.5.5 (appearance states); §7.3.5 (a name holds no null byte).