minor: More comments to AggregateMode::PartialReduce (#22800)

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Author: 2010YOUY01

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

@@ -211,6 +211,12 @@ pub enum AggregateMode {
     ///     /         \      /         \
     ///  Partial   Partial  Partial   Partial
     /// ```
+    ///
+    /// # Motivation
+    ///
+    /// This reduces shuffling traffic in a distributed setting. See
+    /// <https://github.com/datafusion-contrib/datafusion-distributed/issues/360>
+    /// for details.
     PartialReduce,
 }