Include an example of spark.*.extraJavaOptions

Right now it's not super obvious how to specify multiple
java options, especially ones with white spaces.

Author: andrewor14

conf/spark-defaults.conf.template

@@ -2,7 +2,8 @@
 # This is useful for setting default environmental settings.
 
 # Example:
-# spark.master            spark://master:7077
-# spark.eventLog.enabled  true
-# spark.eventLog.dir      hdfs://namenode:8021/directory
-# spark.serializer        org.apache.spark.serializer.KryoSerializer
+# spark.master                     spark://master:7077
+# spark.eventLog.enabled           true
+# spark.eventLog.dir               hdfs://namenode:8021/directory
+# spark.serializer                 org.apache.spark.serializer.KryoSerializer
+# spark.executor.extraJavaOptions  -XX:+PrintGCDetail -Dmy.property="one two three"