Atomic writer's semantics and wording change proposal. 

This is a proposed wording change per my comment on this ticket:
https://issues.apache.org/jira/browse/HADOOP-9371?focusedCommentId=13625721&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13625721


A file may have multiple writers and each writers only guarantee on consistency is during a sync(...) call.