Cross-link SliverFixedExtentList/SliverPrototypeExtentList/SliverVariedExtentList (#143184)

Fixes https://github.com/flutter/flutter/issues/141678.

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

@@ -381,6 +381,8 @@ class SliverList extends SliverMultiBoxAdaptorWidget {
 ///  * [SliverPrototypeExtentList], which is similar to [SliverFixedExtentList]
 ///    except that it uses a prototype list item instead of a pixel value to define
 ///    the main axis extent of each item.
+///  * [SliverVariedExtentList], which supports children with varying (but known
+///    upfront) extents.
 ///  * [SliverFillViewport], which determines the [itemExtent] based on
 ///    [SliverConstraints.viewportMainAxisExtent].
 ///  * [SliverList], which does not require its children to have the same

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

@@ -28,6 +28,8 @@ import 'sliver.dart';
 ///
 ///  * [SliverFixedExtentList], whose children are forced to a given pixel
 ///    extent.
+///  * [SliverVariedExtentList], which supports children with varying (but known
+///    upfront) extents.
 ///  * [SliverList], which does not require its children to have the same
 ///    extent in the main axis.
 ///  * [SliverFillViewport], which sizes its children based on the

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

@@ -27,6 +27,9 @@ import 'sliver.dart';
 ///
 ///  * [SliverFixedExtentList], whose children are forced to a given pixel
 ///    extent.
+///  * [SliverPrototypeExtentList], which is similar to [SliverFixedExtentList]
+///    except that it uses a prototype list item instead of a pixel value to define
+///    the main axis extent of each item.
 ///  * [SliverList], which does not require its children to have the same
 ///    extent in the main axis.
 ///  * [SliverFillViewport], which sizes its children based on the