code: def set_pkg_desc(self, doc, text):
          
          self.assert_package_exists()
          if not self.package_desc_set:
              self.package_desc_set = True
              if validations.validate_pkg_desc(text):
                  doc.package.description = str_from_text(text)
              else:
                  raise SPDXValueError('Package::Description')
          else:
              raise CardinalityError('Package::Description')
docs: Set's the package's description.
          Raises SPDXValueError if text is not free form text.
          Raises CardinalityError if description already set.
          Raises OrderError if no package previously defined.