improve index sorter tests (rename the two different readers); improve javadocs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1634549 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6591: ZkStateReader.updateClusterState should refresh cluster state for watched collections

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1634554 13f79535-47bb-0310-9956-ffa450edef68