Configures an <code><a>Expression</a></code> for the search domain. Used to create new expressions and modify existing ones. If the expression exists, the new configuration replaces the old one. For more information, see <a href="http://docs.aws.amazon.com/cloudsearch/latest/developerguide/configuring-expressions.html" target="_blank">Configuring Expressions</a> in the <i>Amazon CloudSearch Developer Guide</i>.