Improve documentation of CardTheme.shape (#139096)

The previous documentation did not point out the difference between the default corner radius for Material 2 and Material 3.

Fixes: https://github.com/flutter/flutter/issues/139093

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

@@ -164,7 +164,8 @@ class Card extends StatelessWidget {
   ///
   /// If this property is null then [CardTheme.shape] of [ThemeData.cardTheme]
   /// is used. If that's null then the shape will be a [RoundedRectangleBorder]
-  /// with a circular corner radius of 4.0.
+  /// with a circular corner radius of 12.0 and if [ThemeData.useMaterial3] is
+  /// false, then the circular corner radius will be 4.0.
   final ShapeBorder? shape;
 
   /// Whether to paint the [shape] border in front of the [child].

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

@@ -78,7 +78,8 @@ class CardTheme with Diagnosticable {
   /// Overrides the default value for [Card.shape].
   ///
   /// If null, [Card] then uses a [RoundedRectangleBorder] with a circular
-  /// corner radius of 4.0.
+  /// corner radius of 12.0 and if [ThemeData.useMaterial3] is false,
+  /// then the circular corner radius will be 4.0.
   final ShapeBorder? shape;
 
   /// Creates a copy of this object with the given fields replaced with the