[MINOR][DOC] Add an up-to-date description for default serialization during shuffling

## What changes were proposed in this pull request?

This PR aims to make the doc up-to-date. The documentation is generally correct, but after https://issues.apache.org/jira/browse/SPARK-13926, Spark starts to choose Kyro as a default serialization library during shuffling of simple types, arrays of simple types, or string type.

## How was this patch tested?

This is a documentation update.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15315 from dongjoon-hyun/SPARK-DOC-SERIALIZER.

docs/tuning.md

@@ -45,6 +45,7 @@ and calling `conf.set("spark.serializer", "org.apache.spark.serializer.KryoSeria
 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.
+Since Spark 2.0.0, we internally use Kryo serializer when shuffling RDDs with simple types, arrays of simple types, or string type.
 
 Spark automatically includes Kryo serializers for the many commonly-used core Scala classes covered
 in the AllScalaRegistrar from the [Twitter chill](https://github.com/twitter/chill) library.