[Material] Expand BottomNavigationBar API (reprise) (#28159)

examples/flutter_gallery/lib/demo/material/bottom_navigation_demo.dart

@@ -189,6 +189,7 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
           .toList(),
       currentIndex: _currentIndex,
       type: _type,
+      //iconSize: 4.0,
       onTap: (int index) {
         setState(() {
           _navigationViews[_currentIndex].controller.reverse();

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

@@ -9,8 +9,8 @@ import 'framework.dart';
 /// An interactive button within either material's [BottomNavigationBar]
 /// or the iOS themed [CupertinoTabBar] with an icon and title.
 ///
-/// This class is rarely used in isolation. Commonly embedded in one of the
-/// bottom navigation widgets above.
+/// This class is rarely used in isolation. It is typically embedded in one of
+/// the bottom navigation widgets above.
 ///
 /// See also:
 ///
@@ -67,7 +67,7 @@ class BottomNavigationBarItem {
   ///
   /// If the navigation bar's type is [BottomNavigationBarType.shifting], then
   /// the entire bar is flooded with the [backgroundColor] when this item is
-  /// tapped.
+  /// tapped. This will override [BottomNavigationBar.backgroundColor].
   ///
   /// Not used for [CupertinoTabBar]. Control the invariant bar color directly
   /// via [CupertinoTabBar.backgroundColor].