HADOOP-9894. Merging change r1518420 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1518421 13f79535-47bb-0310-9956-ffa450edef68
YARN-1034. Remove experimental in the Fair Scheduler documentation. (Karthik Kambatla via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1518445 13f79535-47bb-0310-9956-ffa450edef68