Returns a pair: (doc_id, absolute filepath of document file)
        for each document in this repository.
        Order is arbitrary.

def iter_doc_filepaths(self, **kwargs):  # pylint: disable=W0613
        """Returns a pair: (doc_id, absolute filepath of document file)
        for each document in this repository.
        Order is arbitrary.
        """
        with self._index_lock:
            for doc_id, info in self._doc_index.items():
                if not self._is_alias(doc_id):
                    yield doc_id, info[-1]