LUCENE-5889: add missing javadocs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1620180 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6424: The hdfs block cache BLOCKCACHE_WRITE_ENABLED is not defaulting to false like it should.
May be related to the SOLR-6423 fail.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1620181 13f79535-47bb-0310-9956-ffa450edef68