HDFS-5625. Write end user documentation for HDFS ACLs. Contributed by Chris Nauroth.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4685@1567421 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5625. Add documentation example for setting a default ACL. Contributed by Chris Nauroth.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4685@1567422 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'HDFS-4685' into HDFS-5925
HDFS-5925: code review feedback to bring back tests related to fsimage and edits