Gets the changes a certification signature permits after it (its DocMDP transform parameters' P), or
null for an approval signature or a document timestamp. Only the document's first signature certifies: a
DocMDP reference in a later one is ignored (§12.8.1).
public PdfCertificationPermissions? CertificationPermissions { get; }
ISO 32000-2 §12.8.1 and §12.8.2.2, Tables 256 and 257.