Code review feedback.

Author: rxin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -701,6 +701,9 @@ object ConvertToLocalRelation extends Rule[LogicalPlan] {
 
 /**
  * Replaces logical [[Distinct]] operator with an [[Aggregate]] operator.
+ * {{{
+ *   SELECT DISTINCT f1, f2 FROM t  ==>  SELECT f1, f2 FROM t GROUP BY f1, f2
+ * }}}
  */
 object ReplaceDistinctWithAggregate extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {