LUCENE-5016:  Sampling can break FacetResult labeling

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1487808 13f79535-47bb-0310-9956-ffa450edef68
improve javadocs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1487818 13f79535-47bb-0310-9956-ffa450edef68