Broadcast variable visibility change & doc update.

Note that previously Broadcast class was accidentally marked as private[spark]. It needs to be public
for broadcast variables to work. Also exposing the broadcast varaible id.

core/src/main/scala/org/apache/spark/broadcast/Broadcast.scala

@@ -17,13 +17,39 @@
 
 package org.apache.spark.broadcast
 
-import java.io._
 import java.util.concurrent.atomic.AtomicLong
 
 import org.apache.spark._
 
-private[spark]
-abstract class Broadcast[T](private[spark] val id: Long) extends Serializable {
+/**
+ * A broadcast variable. Broadcast variables allow the programmer to keep a read-only variable
+ * cached on each machine rather than shipping a copy of it with tasks. They can be used, for
+ * example, to give every node a copy of a large input dataset in an efficient manner. Spark also
+ * attempts to distribute broadcast variables using efficient broadcast algorithms to reduce
+ * communication cost.
+ *
+ * Broadcast variables are created from a variable `v` by calling [[SparkContext#broadcast]].
+ * The broadcast variable is a wrapper around `v`, and its value can be accessed by calling the
+ * `value` method. The interpreter session below shows this:
+ *
+ * {{{
+ * scala> val broadcastVar = sc.broadcast(Array(1, 2, 3))
+ * broadcastVar: spark.Broadcast[Array[Int]] = spark.Broadcast(b5c40191-a864-4c7d-b9bf-d87e1a4e787c)
+ *
+ * scala> broadcastVar.value
+ * res0: Array[Int] = Array(1, 2, 3)
+ * }}}
+ *
+ * After the broadcast variable is created, it should be used instead of the value `v` in any
+ * functions run on the cluster so that `v` is not shipped to the nodes more than once.
+ * In addition, the object `v` should not be modified after it is broadcast in order to ensure
+ * that all nodes get the same value of the broadcast variable (e.g. if the variable is shipped
+ * to a new node later).
+ *
+ * @param id A unique identifier for the broadcast variable.
+ * @tparam T Type of the data contained in the broadcast variable.
+ */
+abstract class Broadcast[T](val id: Long) extends Serializable {
   def value: T
 
   // We cannot have an abstract readObject here due to some weird issues with

core/src/main/scala/org/apache/spark/broadcast/package.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark
+
+/**
+ * Package for broadcast variables. See [[broadcast.Broadcast]] for details.
+ */
+package object broadcast {
+  // For package docs only
+}