Improve the documentation of Card (#178834)

The documentation of `Card` isn't clear enough on some properties and
the differences between the constructors.

Fixes https://github.com/flutter/flutter/issues/178611.

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---------

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

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

@@ -47,9 +47,16 @@ enum _CardVariant { elevated, filled, outlined }
 /// ** See code in examples/api/lib/material/card/card.1.dart **
 /// {@end-tool}
 ///
-/// Material Design 3 introduced new types of cards. The default [Card] is the
-/// elevated card. To create a filled card, use [Card.filled]; to create a outlined
-/// card, use [Card.outlined].
+/// For Material Design 2 (when [ThemeData.useMaterial3] is false), there is a
+/// single card type: the elevated card. In that mode the named constructors
+/// ([Card.filled], [Card.outlined]) behave the same as the default [Card].
+///
+/// For Material Design 3 (when [ThemeData.useMaterial3] is true), three visual
+/// variants are available: the default [Card] (elevated), [Card.filled], and
+/// [Card.outlined]. All variants share the same theme class, [CardThemeData],
+/// so theme properties (for example [CardThemeData.shape]) apply to every card
+/// variant within the theme's scope.
+///
 /// {@tool dartpad}
 /// This sample shows creation of [Card] widgets for elevated, filled and
 /// outlined types, as described in: https://m3.material.io/components/cards/overview
@@ -64,7 +71,10 @@ enum _CardVariant { elevated, filled, outlined }
 ///  * <https://material.io/design/components/cards.html>
 ///  * <https://m3.material.io/components/cards>
 class Card extends StatelessWidget {
-  /// Creates a Material Design card.
+  /// Creates an elevated variant of Card.
+  ///
+  /// Elevated cards have a drop shadow, providing more separation from the
+  /// background than filled cards, but less than outlined cards.
   ///
   /// The [elevation] must be null or non-negative.
   const Card({
@@ -85,7 +95,10 @@ class Card extends StatelessWidget {
   /// Create a filled variant of Card.
   ///
   /// Filled cards provide subtle separation from the background. This has less
-  /// emphasis than elevated(default) or outlined cards.
+  /// emphasis than elevated cards (the default) or outlined cards.
+  ///
+  /// If [ThemeData.useMaterial3] is false, this constructor is equivalent to
+  /// the default constructor of [Card].
   const Card.filled({
     super.key,
     this.color,
@@ -105,6 +118,15 @@ class Card extends StatelessWidget {
   ///
   /// Outlined cards have a visual boundary around the container. This can
   /// provide greater emphasis than the other types.
+  ///
+  /// The card's outline is defined by the [shape] property. By default, the
+  /// card uses a [RoundedRectangleBorder] with a 12.0 corner radius, a 1.0
+  /// border width, and the color from [ColorScheme.outlineVariant]. If you
+  /// provide a custom [shape], it is recommended to use an [OutlinedBorder]
+  /// with a non-null [OutlinedBorder.side] to keep a visible outline.
+  ///
+  /// If [ThemeData.useMaterial3] is false, this constructor is equivalent to
+  /// the default constructor of [Card].
   const Card.outlined({
     super.key,
     this.color,
@@ -166,10 +188,11 @@ class Card extends StatelessWidget {
   ///
   /// Defines the card's [Material.shape].
   ///
-  /// If this property is null then the ambient [CardThemeData.shape] is used.
-  /// If that's null then the shape will be a [RoundedRectangleBorder]
-  /// with a circular corner radius of 12.0 and if [ThemeData.useMaterial3] is
-  /// false, then the circular corner radius will be 4.0.
+  /// If null, the ambient [CardTheme.shape] from [ThemeData.cardTheme] is used.
+  /// If that is also null, the shape defaults to a [RoundedRectangleBorder].
+  /// The default corner radius is 12.0 when [ThemeData.useMaterial3] is true,
+  /// and 4.0 otherwise. For Material 3 outlined cards, the default [shape] also
+  /// includes a border side (see [OutlinedBorder.side]).
   final ShapeBorder? shape;
 
   /// Whether to paint the [shape] border in front of the [child].