Update material spec references in BottomSheet et al., Scaffold (#26042)

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

@@ -40,9 +40,11 @@ const double _kCloseProgressThreshold = 0.5;
 ///
 /// See also:
 ///
-///  * [ScaffoldState.showBottomSheet]
-///  * [showModalBottomSheet]
-///  * <https://material.google.com/components/bottom-sheets.html>
+///  * [showBottomSheet] and [ScaffoldState.showBottomSheet], for showing
+///    non-modal "persistent" bottom sheets.
+///  * [showModalBottomSheet], which can be used to display a modal bottom
+///    sheet.
+///  * <https://material.io/design/components/sheets-bottom.html>
 class BottomSheet extends StatefulWidget {
   /// Creates a bottom sheet.
   ///
@@ -319,7 +321,7 @@ class _ModalBottomSheetRoute<T> extends PopupRoute<T> {
 ///    passed as the `builder` argument to [showModalBottomSheet].
 ///  * [showBottomSheet] and [ScaffoldState.showBottomSheet], for showing
 ///    non-modal bottom sheets.
-///  * <https://material.google.com/components/bottom-sheets.html#bottom-sheets-modal-bottom-sheets>
+///  * <https://material.io/design/components/sheets-bottom.html#modal-bottom-sheet>
 Future<T> showModalBottomSheet<T>({
   @required BuildContext context,
   @required WidgetBuilder builder,
@@ -370,7 +372,7 @@ Future<T> showModalBottomSheet<T>({
 ///  * [showModalBottomSheet], which can be used to display a modal bottom
 ///    sheet.
 ///  * [Scaffold.of], for information about how to obtain the [BuildContext].
-///  * <https://material.google.com/components/bottom-sheets.html#bottom-sheets-persistent-bottom-sheets>
+///  * <https://material.io/design/components/sheets-bottom.html#standard-bottom-sheet>
 PersistentBottomSheetController<T> showBottomSheet<T>({
   @required BuildContext context,
   @required WidgetBuilder builder,

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

@@ -779,10 +779,6 @@ class Scaffold extends StatefulWidget {
   ///
   /// The [persistentFooterButtons] are rendered above the
   /// [bottomNavigationBar] but below the [body].
-  ///
-  /// See also:
-  ///
-  ///  * <https://material.google.com/components/buttons.html#buttons-persistent-footer-buttons>
   final List<Widget> persistentFooterButtons;
 
   /// A panel displayed to the side of the [body], often hidden on mobile
@@ -1336,7 +1332,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   ///  * [showModalBottomSheet], which can be used to display a modal bottom
   ///    sheet.
   ///  * [Scaffold.of], for information about how to obtain the [ScaffoldState].
-  ///  * <https://material.google.com/components/bottom-sheets.html#bottom-sheets-persistent-bottom-sheets>
+  ///  * <https://material.io/design/components/sheets-bottom.html#standard-bottom-sheet>
   PersistentBottomSheetController<T> showBottomSheet<T>(WidgetBuilder builder) {
     _closeCurrentBottomSheet();
     final AnimationController controller = BottomSheet.createAnimationController(this)