Add a note about dense union field name

docs/source/format/StatisticsSchema.rst

@@ -139,7 +139,7 @@ Here is the details of the ``map`` of the ``statistics``:
        ``int64`` distinct count statistic and a ``float64`` average
        byte width statistic. See the description of each statistic below.
 
-       We don't standardize field names for the dense union because we
+       Dense union has name for each field but we don't standardize field names for the dense union because we
        can access to proper field by type code not name. So we can use
        any valid name for fields.