Merge r1588258 from trunk, HDFS-6153. Document "fileId" and "childrenNum" fields in the FileStatus Json schema

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1588262 13f79535-47bb-0310-9956-ffa450edef68
Merge 1588281 to branch-2. YARN-1931. Private API change in YARN-1824 in 2.4 broke compatibility with previous releases (Sandy Ryza via tgraves)

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