Add private notes.

        :param private_notes: hidden notes for the current document
        :type private_notes: string

        :param source: source for the given private notes
        :type source: string

def add_private_note(self, private_notes, source=None):
        """Add private notes.

        :param private_notes: hidden notes for the current document
        :type private_notes: string

        :param source: source for the given private notes
        :type source: string
        """
        self._append_to('_private_notes', self._sourced_dict(
            source,
            value=private_notes,
        ))