HDFS-4819. Update Snapshot doc to clarify that nested snapshots are not allowed.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1492062 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-9078-trunk-gd