Clean up comments on LightweightPrefixMap

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439461 13f79535-47bb-0310-9956-ffa450edef68
git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439462 13f79535-47bb-0310-9956-ffa450edef68