HDFS-5685. Merge r1590100 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1590104 13f79535-47bb-0310-9956-ffa450edef68
MAPREDUCE-5639. Port DistCp2 document to trunk (Akira AJISAKA via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1590131 13f79535-47bb-0310-9956-ffa450edef68
YARN-483. Improve documentation on log aggregation in yarn-default.xml (Akira Ajisaka via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1590151 13f79535-47bb-0310-9956-ffa450edef68