plugin parses docstring differently than expected

[dandhlee]

With the given docstring:

        """Union[int, None]: Expiration time in milliseconds for a partition.

        If :attr:`partition_expiration` is set and :attr:`type_` is
        not set, :attr:`type_` will default to
        :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`.
        """

It fails to parse properly and treat the whole syntax as a top line summary when it encounters xref:type_ since we look for :type in the code. This should be prevented by only looking for :type and nothing before/after.

[dandhlee]

Issue first seen in googleapis/python-bigquery#698