Add comment on why interruptOnCancel is in setJobGroup

Author: aarondav

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -390,6 +390,10 @@ class SparkContext(config: SparkConf) extends Logging {
   def setJobGroup(groupId: String, description: String, interruptOnCancel: Boolean = false) {
     setLocalProperty(SparkContext.SPARK_JOB_DESCRIPTION, description)
     setLocalProperty(SparkContext.SPARK_JOB_GROUP_ID, groupId)
+    // Note: Specifying interruptOnCancel in setJobGroup (rather than cancelJobGroup) avoids
+    // changing several public APIs and allows Spark cancellations outside of the cancelJobGroup
+    // APIs to also take advantage of this property (e.g., internal job failures or canceling from
+    // JobProgressTab UI) on a per-job basis.
     setLocalProperty(SparkContext.SPARK_JOB_INTERRUPT_ON_CANCEL, interruptOnCancel.toString)
   }