Add missing doc string to test__sample_too_small test (#2105)

tests/unit/sampling/test_independent_sampler.py

@@ -407,6 +407,7 @@ def _connect_tables(sampled_data):
         assert result == instance._finalize.return_value
 
     def test__sample_too_small(self):
+        """Test that the ``_sample`` method works even when sample size is very small."""
         instance = Mock()
         metadata = Mock()
         metadata.tables = {