Review feedback.

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala

@@ -217,8 +217,8 @@ private[spark] object CoarseGrainedExecutorBackend extends Logging {
       |   --hostname <hostname>
       |   --cores <cores>
       |   --app-id <appid>
-      |   [--worker-url <workerUrl>]
-      |   [--user-class-path <url>]
+      |   --worker-url <workerUrl>
+      |   --user-class-path <url>
       |""".stripMargin)
     System.exit(1)
   }

core/src/main/scala/org/apache/spark/executor/ExecutorURLClassLoader.scala

@@ -29,16 +29,27 @@ import org.apache.spark.util.ParentClassLoader
  * The addURL method in URLClassLoader is protected. We subclass it to make this accessible.
  * We also make changes so user classes can come before the default classes.
  */
-
 private[spark] trait MutableURLClassLoader extends ClassLoader {
   def addURL(url: URL)
   def getURLs: Array[URL]
 }
 
+/**
+ * A class loader that gives preference to its own URLs over the parent class loader
+ * when loading classes and resources.
+ */
 private[spark] class ChildExecutorURLClassLoader(urls: Array[URL], parent: ClassLoader)
   extends URLClassLoader(urls, null) with MutableURLClassLoader {
 
   private val parentClassLoader = new ParentClassLoader(parent)
+
+  /**
+   * Used to implement fine-grained class loading locks similar to what is done by Java 7. This
+   * prevents deadlock issues when using non-hierarchical class loaders.
+   *
+   * Note that due to Java 6 compatibility (and some issues with implementing class loaders in
+   * Scala), Java 7's `ClassLoader.registerAsParallelCapable` method is not called.
+   */
   private val locks = new ConcurrentHashMap[String, Object]()
 
   override def loadClass(name: String, resolve: Boolean): Class[_] = {
@@ -84,6 +95,9 @@ private[spark] class ChildExecutorURLClassLoader(urls: Array[URL], parent: Class
 
 }
 
+/**
+ * A MutableURLClassLoader that follows the standard parent-first hierarchy.
+ */
 private[spark] class ExecutorURLClassLoader(urls: Array[URL], parent: ClassLoader)
   extends URLClassLoader(urls, parent) with MutableURLClassLoader {