[SPARK-20421][core] Add a missing deprecation tag.

In the previous patch I deprecated StorageStatus, but not the
method in SparkContext that exposes that class publicly. So deprecate
the method too.

Author: Marcelo Vanzin

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

@@ -1734,6 +1734,7 @@ class SparkContext(config: SparkConf) extends Logging {
    * Return information about blocks stored in all of the slaves
    */
   @DeveloperApi
+  @deprecated("This method may change or be removed in a future release.", "2.2.0")
   def getExecutorStorageStatus: Array[StorageStatus] = {
     assertNotStopped()
     env.blockManager.master.getStorageStatus