improve upgrading instructions for 4.3 ... grouped by theme, and more focused on what people need to be concerned with instead of just cut/paste the changes entry for each jira. (merge r1469508)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_3@1469510 13f79535-47bb-0310-9956-ffa450edef68