Update code comments for navigation_bar.dart (#162596)

Fixes flutter/flutter#162592

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### Comments error:
```
class NavigationDestination extends StatelessWidget {
...

  /// The text label that appears below the icon of this
  /// [NavigationDestination].
  ///
  /// The accompanying [Text] widget will use
  /// [NavigationBarThemeData.labelTextStyle]. If this are null, the default
  /// text style would use [TextTheme.labelSmall] with [ColorScheme.onSurface].
  final String label;

...
}
```

`If this are null, the default text style would use
[TextTheme.labelSmall]` is wrong. Actually, This is right in material2,
but wrong in material3. I have dived into `_NavigationBarDefaultsM3`, I
found this:
```
class _NavigationBarDefaultsM3 extends NavigationBarThemeData {
...

@OverRide
  MaterialStateProperty<TextStyle?>? get labelTextStyle {
    return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
    final TextStyle style = _textTheme.labelMedium!;
      return style.apply(
        color: states.contains(MaterialState.disabled)
          ? _colors.onSurfaceVariant.withOpacity(0.38)
          : states.contains(MaterialState.selected)
            ? _colors.onSurface
            : _colors.onSurfaceVariant
      );
    });
  }

...
}
```
**The default text style would use [TextTheme.labelMedium] in
Material3.**


### Changes

Just clarify the doc comments above.[test-exempt]


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---------

Co-authored-by: Taha Tesser <tessertaha@gmail.com>

Author: gaspardruan

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

@@ -385,9 +385,12 @@ class NavigationDestination extends StatelessWidget {
   /// The text label that appears below the icon of this
   /// [NavigationDestination].
   ///
-  /// The accompanying [Text] widget will use
-  /// [NavigationBarThemeData.labelTextStyle]. If this are null, the default
-  /// text style would use [TextTheme.labelSmall] with [ColorScheme.onSurface].
+  /// The accompanying [Text] widget will use [NavigationBarThemeData.labelTextStyle].
+  /// If this is null, the default text style will use [TextTheme.labelMedium] with
+  /// [ColorScheme.onSurface] when the destination is selected and
+  /// [ColorScheme.onSurfaceVariant] when the destination is unselected. If
+  /// [ThemeData.useMaterial3] is false, then the default text style will use
+  /// [TextTheme.labelSmall] with [ColorScheme.onSurface].
   final String label;
 
   /// The text to display in the tooltip for this [NavigationDestination], when