doc: More comments on GroupedHashAggregateStream refactor (apache#23200)

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Original discussion: apache#23165

This PR adds more comments to explain the on-going refactor, to reduce
confusion.

- refactor: apache#22710

cc @alamb @Rachelint 

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Author: 2010YOUY01

datafusion/physical-plan/src/aggregates/mod.rs

@@ -1009,6 +1009,13 @@ impl AggregateExec {
             ));
         }
 
+        // `GroupedHashAggregateStream` is being incrementally refactored. See the
+        // tracking issue for details.
+        //
+        // New features and improvements should go directly into the new implementation.
+        // Please coordinate through the tracking issue.
+        //
+        // Issue: <https://github.com/apache/datafusion/issues/22710>
         if context
             .session_config()
             .options()
@@ -1028,7 +1035,7 @@ impl AggregateExec {
             }
         }
 
-        // grouping by something else and we need to just materialize all results
+        // Execution paths that have not been migrated use the fallback implementation
         Ok(StreamType::GroupedHash(GroupedHashAggregateStream::new(
             self, context, partition,
         )?))

datafusion/physical-plan/src/aggregates/row_hash.rs

@@ -132,6 +132,16 @@ enum OutOfMemoryMode {
 
 /// HashTable based Grouping Aggregator
 ///
+/// # Development Note
+///
+/// This implementation is being incrementally refactored. See the tracking issue
+/// for details.
+///
+/// New features and improvements should go directly into the new implementation.
+/// Please coordinate through the tracking issue.
+///
+/// Issue: <https://github.com/apache/datafusion/issues/22710>
+///
 /// # Design Goals
 ///
 /// This structure is designed so that updating the aggregates can be