Updated doc for Limit.

Author: rxin

sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala

@@ -63,12 +63,14 @@ case class Union(children: Seq[SparkPlan])(@transient sc: SparkContext) extends
 }
 
 /**
- * Take the first limit elements.
+ * Take the first limit elements. Note that the implementation is different depending on whether
+ * this is a terminal operator or not. If it is terminal and is invoked using executeCollect,
+ * this operator uses Spark's take method on the Spark driver. If it is not terminal or is
+ * invoked using execute, we first take the limit on each partition, and then repartition all the
+ * data to a single partition to compute the global limit.
  */
 case class Limit(limit: Int, child: SparkPlan)(@transient sc: SparkContext) extends UnaryNode {
   override def otherCopyArgs = sc :: Nil
-  // Note that the implementation is different depending on
-  // whether this is a terminal operator or not.
 
   override def output = child.output