code: def set_doc_comment(self, doc, comment):
          
          if not self.doc_comment_set:
              self.doc_comment_set = True
              if validations.validate_doc_comment(comment):
                  doc.comment = str_from_text(comment)
                  return True
              else:
                  raise SPDXValueError('Document::Comment')
          else:
              raise CardinalityError('Document::Comment')
docs: Sets document comment, Raises CardinalityError if
          comment already set.
          Raises SPDXValueError if comment is not free form text.