add docs

Author: jose-torres

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousCoalesceExec.scala

@@ -27,6 +27,11 @@ import org.apache.spark.sql.catalyst.plans.physical.{Partitioning, SinglePartiti
 import org.apache.spark.sql.execution.SparkPlan
 import org.apache.spark.sql.execution.streaming.continuous.shuffle.{ContinuousShuffleReadPartition, ContinuousShuffleReadRDD}
 
+/**
+ * Physical plan for coalescing a continuous processing plan.
+ *
+ * Currently, only coalesces to a single partition are supported. `numPartitions` must be 1.
+ */
 case class ContinuousCoalesceExec(numPartitions: Int, child: SparkPlan) extends SparkPlan {
   override def output: Seq[Attribute] = child.output
 

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousCoalesceRDD.scala

@@ -27,6 +27,10 @@ case class ContinuousCoalesceRDDPartition(index: Int) extends Partition {
   private[continuous] var writersInitialized: Boolean = false
 }
 
+/**
+ * RDD for continuous coalescing. Asynchronously writes all partitions of `prev` into a local
+ * continuous shuffle, and then reads them in the task thread using `reader`.
+ */
 class ContinuousCoalesceRDD(var reader: ContinuousShuffleReadRDD, var prev: RDD[InternalRow])
   extends RDD[InternalRow](reader.context, Nil) {