Create and cache the full path for this document.

        Of the form:

            ``projects/{project_id}/databases/{database_id}/...
                  documents/{document_path}``

        Returns:
            str: The full document path.

        Raises:
            ValueError: If the current document reference has no ``client``.

def _document_path(self):
        """Create and cache the full path for this document.

        Of the form:

            ``projects/{project_id}/databases/{database_id}/...
                  documents/{document_path}``

        Returns:
            str: The full document path.

        Raises:
            ValueError: If the current document reference has no ``client``.
        """
        if self._document_path_internal is None:
            if self._client is None:
                raise ValueError("A document reference requires a `client`.")
            self._document_path_internal = _get_document_path(self._client, self._path)

        return self._document_path_internal