DropdownButton.style API doc example for differing button and menu item text styles (#41650)

* Add dropdown button style doc example for unique text styles

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

@@ -710,6 +710,51 @@ class DropdownButton<T> extends StatefulWidget {
   /// The text style to use for text in the dropdown button and the dropdown
   /// menu that appears when you tap the button.
   ///
+  /// To use a separate text style for selected item when it's displayed within
+  /// the dropdown button,, consider using [selectedItemBuilder].
+  ///
+  /// {@tool snippet --template=stateful_widget_scaffold}
+  ///
+  /// This sample shows a `DropdownButton` with a dropdown button text style
+  /// that is different than its menu items.
+  ///
+  /// ```dart
+  /// List<String> options = <String>['One', 'Two', 'Free', 'Four'];
+  /// String dropdownValue = 'One';
+  ///
+  /// @override
+  /// Widget build(BuildContext context) {
+  ///   return Container(
+  ///     alignment: Alignment.center,
+  ///     color: Colors.blue,
+  ///     child: DropdownButton<String>(
+  ///       value: dropdownValue,
+  ///       onChanged: (String newValue) {
+  ///         setState(() {
+  ///           dropdownValue = newValue;
+  ///         });
+  ///       },
+  ///       style: TextStyle(color: Colors.blue),
+  ///       selectedItemBuilder: (BuildContext context) {
+  ///         return options.map((String value) {
+  ///           return Text(
+  ///             dropdownValue,
+  ///             style: TextStyle(color: Colors.white),
+  ///           );
+  ///         }).toList();
+  ///       },
+  ///       items: options.map<DropdownMenuItem<String>>((String value) {
+  ///         return DropdownMenuItem<String>(
+  ///           value: value,
+  ///           child: Text(value),
+  ///         );
+  ///       }).toList(),
+  ///     ),
+  ///   );
+  /// }
+  /// ```
+  /// {@end-tool}
+  ///
   /// Defaults to the [TextTheme.subhead] value of the current
   /// [ThemeData.textTheme] of the current [Theme].
   final TextStyle style;