@Override
    public void serialize(Field field, FieldWriter writer) {
        writer.write(field, this);
    }
Creates a reference pointing to a particular document instance in the document
type referenced by <code>referenceType</code>.

@param referenceType reference target type
@param documentId document ID of the same document type as that given by <code>referenceType</code>
@throws IllegalArgumentException if documentId is not of the expected document type
document/src/main/java/com/yahoo/document/datatypes/ReferenceFieldValue.java