Add javadoc note to LeafCollector#finish (#12643)

lucene/core/src/java/org/apache/lucene/search/LeafCollector.java

@@ -125,6 +125,9 @@ public interface LeafCollector {
    * is typically useful to compile data that has been collected on this leaf, e.g. to convert facet
    * counts on leaf ordinals to facet counts on global ordinals. The default implementation does
    * nothing.
+   *
+   * <p>Note: It can be assumed that this method will only be called once per LeafCollector
+   * instance.
    */
   default void finish() throws IOException {}
 }