Gets or sets the field flags (Ff) the caller chooses: those common to all fields and those of the field's type. The
flags that select the type (Radio, Pushbutton, Combo) are set by the Add method; the default is none.
public PdfFieldFlags Flags { get; set; }
ISO 32000-2 §12.7.4.1, Table 227; §12.7.5.2.1, Table 229; §12.7.5.3, Table 231; §12.7.5.4, Table 233.