Add warning about using MergeCursor in CursorAdapter

/**

 * Adapter that exposes data from a {@link android.database.Cursor Cursor} to a

 * {@link android.widget.ListView ListView} widget. The Cursor must include 

 * a column named "_id" or this class will not work.

 * {@link android.widget.ListView ListView} widget.

 * <p>

 * The Cursor must include a column named "_id" or this class will not work.

 * Additionally, using {@link android.database.MergeCursor} with this class will

 * not work if the merged Cursors have overlapping values in their "_id"

 * columns.

 */

public abstract class CursorAdapter extends BaseAdapter implements Filterable,

        CursorFilter.CursorFilterClient {