LUCENE-672: added description of the 'under-merging' bug that was also fixed with this change
LUCENE-672: added description of the 'under-merging' bug that was also fixed with this change

git-svn-id: 4c5078813df38efa56971a28e09a55254294f104@480238 13f79535-47bb-0310-9956-ffa450edef68

apache/solr,apache/solr,apache/solr,apache/solr,apache/solr