svn merge -c 1492062 from trunk for HDFS-4819. Update Snapshot doc to clarify that nested snapshots are not allowed.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1492064 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-MAPREDUCE-4884-branch-2-gd