document laziness of parallelize

Took me several hours to figure out this behavior. It would be good to highlight it in the documentation.

Author: Ariel Rabkin <[email protected]>

Closes apache#1070 from asrabkin/master and squashes the following commits:

29a076e [Ariel Rabkin] doc fix

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

@@ -434,12 +434,21 @@ class SparkContext(config: SparkConf) extends Logging {
 
   // Methods for creating RDDs
 
-  /** Distribute a local Scala collection to form an RDD. */
+  /** Distribute a local Scala collection to form an RDD.
+   *
+   * @note Parallelize acts lazily. If `seq` is a mutable collection and is
+   * altered after the call to parallelize and before the first action on the
+   * RDD, the resultant RDD will reflect the modified collection. Pass a copy of
+   * the argument to avoid this.
+   */
   def parallelize[T: ClassTag](seq: Seq[T], numSlices: Int = defaultParallelism): RDD[T] = {
     new ParallelCollectionRDD[T](this, seq, numSlices, Map[Int, Seq[String]]())
   }
 
-  /** Distribute a local Scala collection to form an RDD. */
+  /** Distribute a local Scala collection to form an RDD.
+   *
+   * This method is identical to `parallelize`.
+   */
   def makeRDD[T: ClassTag](seq: Seq[T], numSlices: Int = defaultParallelism): RDD[T] = {
     parallelize(seq, numSlices)
   }