docs: Updates to API docs based on usability study feedback. (#6126)

packages/flutter/lib/src/material/list_item.dart

@@ -25,6 +25,7 @@ import 'theme.dart';
 ///
 ///  * [MaterialList]
 ///  * [CircleAvatar]
+///  * [Divider]
 ///  * <https://www.google.com/design/spec/components/lists.html>
 class ListItem extends StatelessWidget {
   /// Creates a list item.

packages/flutter/lib/src/widgets/image.dart

@@ -43,6 +43,10 @@ ImageConfiguration createLocalImageConfiguration(BuildContext context, { Size si
 ///
 /// The image is painted using [paintImage], which describes the meanings of the
 /// various fields on this class in more detail.
+///
+/// See also:
+///
+///  * [Icon]
 class Image extends StatefulWidget {
   /// Creates a widget that displays an image.
   ///