Update documentation of `AlertDialog`'s default `TextStyle` for Material 3 (#144697)

## Update `AlertDialog` Documentation for Material 3 Defaults

This pull request updates the documentation for `AlertDialog` to accurately reflect the default text styles (`titleTextStyle` and `contentTextStyle`) when using Material 3.
Previously, the documentation suggested default styles (`TextTheme.titleLarge` for titles and `TextTheme.titleMedium` for content) that do not align with Material 3 implementation, which uses `TextTheme.headlineSmall` and `TextTheme.bodyMedium` respectively.

Fixes #144489

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

@@ -508,7 +508,8 @@ class AlertDialog extends StatelessWidget {
   /// Style for the text in the [title] of this [AlertDialog].
   ///
   /// If null, [DialogTheme.titleTextStyle] is used. If that's null, defaults to
-  /// [TextTheme.titleLarge] of [ThemeData.textTheme].
+  /// [TextTheme.headlineSmall] of [ThemeData.textTheme] if
+  /// [ThemeData.useMaterial3] is true, [TextTheme.titleLarge] otherwise.
   final TextStyle? titleTextStyle;
 
   /// The (optional) content of the dialog is displayed in the center of the
@@ -542,7 +543,8 @@ class AlertDialog extends StatelessWidget {
   /// Style for the text in the [content] of this [AlertDialog].
   ///
   /// If null, [DialogTheme.contentTextStyle] is used. If that's null, defaults
-  /// to [TextTheme.titleMedium] of [ThemeData.textTheme].
+  /// to [TextTheme.bodyMedium] of [ThemeData.textTheme] if
+  /// [ThemeData.useMaterial3] is true, [TextTheme.titleMedium] otherwise.
   final TextStyle? contentTextStyle;
 
   /// The (optional) set of actions that are displayed at the bottom of the