4.3 CHANGES fix up: consolidate multiple 'API Changes' sections and move to just below backcompat so it's more obvious (merge r1469600)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_3@1469606 13f79535-47bb-0310-9956-ffa450edef68