SOLR-6048 the assert was not really failing the test

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1602139 13f79535-47bb-0310-9956-ffa450edef68
add comments to clarify code

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1602141 13f79535-47bb-0310-9956-ffa450edef68