Updated doc for spark.closure.serializer to indicate only Java serializer work.

See discussion from http://apache-spark-developers-list.1001551.n3.nabble.com/bug-using-kryo-as-closure-serializer-td6473.html

Author: Reynold Xin <rxin@apache.org>

Closes #642 from rxin/docs-ser and squashes the following commits:

a507db5 [Reynold Xin] Use "Java" instead of default.
5eb8cdd [Reynold Xin] Updated doc for spark.closure.serializer to indicate only the default serializer work.