Style fixes.

core/src/main/scala/org/apache/spark/util/ThreadUtils.scala

@@ -161,8 +161,14 @@ private[spark] object ThreadUtils {
    * Construct a new Java ForkJoinPool with a specified max parallelism and name prefix.
    */
   def newForkJoinPool(prefix: String, maxThreadNumber: Int): ForkJoinPool = {
+    /**
+     * Extend ForkJoinWorkerThread so we can instantiate it.
+     */
     class SparkForkJoinWorkerThread(pool: ForkJoinPool) extends ForkJoinWorkerThread(pool) {
     }
+    /**
+     * Custom ForkJoinWorkerThreadFactory to specify the thread name.
+     */
     class SparkForkJoinFactory extends ForkJoinPool.ForkJoinWorkerThreadFactory {
       override def newThread(pool: ForkJoinPool): ForkJoinWorkerThread = {
         val th = new SparkForkJoinWorkerThread(pool)
@@ -171,6 +177,9 @@ private[spark] object ThreadUtils {
       }
     }
     val factory = new SparkForkJoinFactory()
-    new ForkJoinPool(maxThreadNumber, factory, null /* handler */, false /* asyncMode */)
+    new ForkJoinPool(maxThreadNumber, factory,
+      null, // handler
+      false // asyncMode
+    )
   }
 }