YARN-1891. Added documentation for NodeManager health-monitoring. Contributed by Varun Vasudev.
svn merge --ignore-ancestry -c 1582891 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1582894 13f79535-47bb-0310-9956-ffa450edef68