[MCHANGELOG-46] Using the dateFormat option the generate changelog report shows a wrong date.

o Clarified the documentation for the dateFormat parameter, so that users are less likely to try to use it for presentation. If dateFormat is set to another format than what is used by the underlying SCM, malformed dates will appear in the report.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk/maven-changelog-plugin@437751 13f79535-47bb-0310-9956-ffa450edef68