Atomic update is a very new feature coming in 4.0 (i.e. grab a recent nightly build to try it out).
It's not documented yet, but here's the JIRA issue: https://issues.apache.org/jira/browse/SOLR-139?focusedCommentId=13269007&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13269007 -Yonik http://lucidimagination.com
