SOLR-6048 the assert was not really failing the test

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1602138 13f79535-47bb-0310-9956-ffa450edef68
add comments to clarify code

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1602140 13f79535-47bb-0310-9956-ffa450edef68