YARN-700. Merging change r1492383 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1492386 13f79535-47bb-0310-9956-ffa450edef68
YARN-648. FS: Add documentation for pluggable policy. (kkambatl via tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1492389 13f79535-47bb-0310-9956-ffa450edef68