Add comments to physical optimizer tests (#14075)

Author: alamb

datafusion/core/tests/physical_optimizer/combine_partial_final_agg.rs

@@ -15,6 +15,10 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! Tests for [`CombinePartialFinalAggregate`] physical optimizer rule
+//!
+//! Note these tests are not in the same module as the optimizer pass because
+//! they rely on `ParquetExec` which is in the core crate.
 use std::sync::Arc;
 
 use arrow::datatypes::{DataType, Field, Schema, SchemaRef};

datafusion/core/tests/physical_optimizer/limited_distinct_aggregation.rs

@@ -15,8 +15,10 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! Tests for the limited distinct aggregation optimizer rule
-
+//! Tests for [`LimitedDistinctAggregation`] physical optimizer rule
+//!
+//! Note these tests are not in the same module as the optimizer pass because
+//! they rely on `ParquetExec` which is in the core crate.
 use super::test_util::{parquet_exec_with_sort, schema, trim_plan_display};
 
 use std::sync::Arc;