LUCENE-5878 - merged javadoc fix to branch 4.x

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1616756 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6336: Cache children as well so that they can be returned when the watcher is reused.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1616772 13f79535-47bb-0310-9956-ffa450edef68