HBASE-9266 Javadoc: Document that HBaseAdmin.flush(...) is synchronous

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1515676 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-HBASE-8559-0.94-gd