HDFS-6007. Update documentation about short-circuit local reads (iwasakims via cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1578995 13f79535-47bb-0310-9956-ffa450edef68
Merge 1579004 and 1579005 from trunk for HDFS-6068. Disallow snapshot names that are also invalid directory names.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1579007 13f79535-47bb-0310-9956-ffa450edef68
YARN-1705. Reset cluster-metrics on transition to standby. (Rohith via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1579015 13f79535-47bb-0310-9956-ffa450edef68