merge HADOOP-9221 from trunk. Convert remaining xdocs to APT.


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Move HADOOP-9221 to correct section in CHANGES.txt.


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HDFS-3598. WebHDFS support for file concat. Contributed by Plamen Jeliazkov.

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YARN-343. Capacity Scheduler maximum-capacity value -1 is invalid (Xuan Gong via tgraves)

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