added a note about dirty graph not supported

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1469777 13f79535-47bb-0310-9956-ffa450edef68
improved description

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1469778 13f79535-47bb-0310-9956-ffa450edef68