SectionIndexer: Update Javadoc for ExpandableListAdapters

SectionIndexer could be implemented by ExpandableListAdapters
that considers groups and sections as synonymous and such
adapters have to account for collapsed groups and should
return appropriate section/position.

Change-Id: If0dec268da8fa30bd26f4d0aaad7e644a9944313

core/java/android/widget/SectionIndexer.java

@@ -20,7 +20,9 @@ package android.widget;
  * Interface that should be implemented on Adapters to enable fast scrolling 
  * in an {@link AbsListView} between sections of the list. A section is a group of list items
  * to jump to that have something in common. For example, they may begin with the
- * same letter or they may be songs from the same artist. 
+ * same letter or they may be songs from the same artist. ExpandableListAdapters that
+ * consider groups and sections as synonymous should account for collapsed groups and return
+ * an appropriate section/position.
  */
 public interface SectionIndexer {
     /**