Get a reference to a document in a collection.

        For a top-level document:

        .. code-block:: python

            >>> client.document('collek/shun')
            >>> # is the same as
            >>> client.document('collek', 'shun')

        For a document in a sub-collection:

        .. code-block:: python

            >>> client.document('mydocs/doc/subcol/child')
            >>> # is the same as
            >>> client.document('mydocs', 'doc', 'subcol', 'child')

        Documents in sub-collections can be nested deeper in a similar fashion.

        Args:
            document_path (Tuple[str, ...]): Can either be

                * A single ``/``-delimited path to a document
                * A tuple of document path segments

        Returns:
            ~.firestore_v1beta1.document.DocumentReference: A reference
            to a document in a collection.

def document(self, *document_path):
        """Get a reference to a document in a collection.

        For a top-level document:

        .. code-block:: python

            >>> client.document('collek/shun')
            >>> # is the same as
            >>> client.document('collek', 'shun')

        For a document in a sub-collection:

        .. code-block:: python

            >>> client.document('mydocs/doc/subcol/child')
            >>> # is the same as
            >>> client.document('mydocs', 'doc', 'subcol', 'child')

        Documents in sub-collections can be nested deeper in a similar fashion.

        Args:
            document_path (Tuple[str, ...]): Can either be

                * A single ``/``-delimited path to a document
                * A tuple of document path segments

        Returns:
            ~.firestore_v1beta1.document.DocumentReference: A reference
            to a document in a collection.
        """
        if len(document_path) == 1:
            path = document_path[0].split(_helpers.DOCUMENT_PATH_DELIMITER)
        else:
            path = document_path

        return DocumentReference(*path, client=self)