HBASE-8076 add better doc for HBaseAdmin#offline API.(Rajesh)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.96@1575582 13f79535-47bb-0310-9956-ffa450edef68
HBASE-10707 Backport parent issue to 0.96

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.96@1575645 13f79535-47bb-0310-9956-ffa450edef68