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


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1440246 13f79535-47bb-0310-9956-ffa450edef68
Move HADOOP-9221 to correct section in CHANGES.txt.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1440247 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'branch-2' of github.com:griddynamics/yhadoop-common into branch-2-gd
Merge branch 'branch-2-gd' of github.com:griddynamics/yhadoop-common into fb-HADOOP-7435-branch-2-gd