TermsQuery and TermsCollector.ToParentBlockJoinQuery, except this
query joins in reverse: you provide a Query matching
parent documents and it joins down to child
documents.IndexWriter.addDocuments(java.util.Collection<org.apache.lucene.document.Document>) or IndexWriter.updateDocuments(org.apache.lucene.index.Term, java.util.Collection<org.apache.lucene.document.Document>) API.