YARN-1452. Added documentation about the configuration and usage of generic application history and the timeline data service. Contributed by Zhijie Shen.
svn merge --ignore-ancestry -c 1581656 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1581658 13f79535-47bb-0310-9956-ffa450edef68