Added spark.shuffle.file.buffer.kb to configuration doc.

Author: Reynold Xin <rxin@apache.org>

== Merge branch commits ==

commit 0eea1d761ff772ff89be234e1e28035d54e5a7de
Author: Reynold Xin <rxin@apache.org>
Date:   Wed Jan 29 14:40:48 2014 -0800

    Added spark.shuffle.file.buffer.kb to configuration doc.