YARN-1080. Improved help message for "yarn logs" command. Contributed by Xuan Gong.
svn merge --ignore-ancestry -c 1518731 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1518734 13f79535-47bb-0310-9956-ffa450edef68