[Docs] actorStream storageLevel default is MEMORY_AND_DISK_SER_2

Comment of the storageLevel param of actorStream says that it defaults to memory-only while the default is MEMORY_AND_DISK_SER_2.

Author: Mario Pastorelli <pastorelli.mario@gmail.com>

Closes #2319 from melrief/master and squashes the following commits:

7b6ce68 [Mario Pastorelli] [Docs] actorStream storageLevel default is MEMORY_AND_DISK_SER_2

streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala

@@ -240,7 +240,7 @@ class StreamingContext private[streaming] (
    * Find more details at: http://spark.apache.org/docs/latest/streaming-custom-receivers.html
    * @param props Props object defining creation of the actor
    * @param name Name of the actor
-   * @param storageLevel RDD storage level. Defaults to memory-only.
+   * @param storageLevel RDD storage level (default: StorageLevel.MEMORY_AND_DISK_SER_2)
    *
    * @note An important point to note:
    *       Since Actor may exist outside the spark framework, It is thus user's responsibility