javadoc: clarify that getDeletedDocs is published safely

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1050222 13f79535-47bb-0310-9956-ffa450edef68

lucene/src/java/org/apache/lucene/index/IndexReader.java

@@ -1241,6 +1241,8 @@ public abstract class IndexReader implements Cloneable,Closeable {
    *  method should return null when there are no deleted
    *  docs.
    *
+   *  The returned instance has been safely published for use by
+   *  multiple threads without additional synchronization.
    * @lucene.experimental */
   public abstract Bits getDeletedDocs();