Update tuning.md

Clarify when serializer is used based on recent user@ mailing list discussion.

docs/tuning.md

@@ -39,7 +39,8 @@ in your operations) and performance. It provides two serialization libraries:
   for best performance.
 
 You can switch to using Kryo by calling `System.setProperty("spark.serializer", "org.apache.spark.serializer.KryoSerializer")`
-*before* creating your SparkContext. The only reason it is not the default is because of the custom
+*before* creating your SparkContext. This setting configures the serializer used for not only shuffling data between worker
+nodes but also when serializing RDDs to disk.  The only reason Kryo is not the default is because of the custom
 registration requirement, but we recommend trying it in any network-intensive application.
 
 Finally, to register your classes with Kryo, create a public class that extends