add warning about Hits.iterator

git-svn-id: https://svn.apache.org/repos/asf/lucene/java/trunk@164699 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/lucene/search/Hits.java

@@ -118,6 +118,11 @@ public final class Hits {
   /**
    * Returns an {@link Iterator} to navigate the Hits.  Each item returned
    * from {@link Iterator#next()} is a {@link Hit}.
+   * <p>
+   * <b>Caution:</b> Iterate only over the hits needed.  Iterating over all
+   * hits is generally not desirable and may be the source of
+   * performance issues.
+   * </p>
    */
   public Iterator iterator() {
     return new HitIterator(this);