adds an <intro>, <act> or <conclu> token span to the document.

def _add_token_span_to_document(self, span_element):
        """
        adds an <intro>, <act> or <conclu> token span to the document.
        """
        for token in span_element.text.split():
            token_id = self._add_token_to_document(token)
            if span_element.tag == 'act':  # doc can have 0+ acts
                self._add_spanning_relation('act_{}'.format(self.act_count),
                                            token_id)
            else:  # <intro> or <conclu>
                self._add_spanning_relation(span_element.tag, token_id)
        if span_element.tag == 'act':
            self.act_count += 1