[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.