Apply suggestions from code review

Co-authored-by: Matthias Ngeo <[email protected]>

forui/lib/src/widgets/select_group/select_group.dart

@@ -4,7 +4,7 @@ import 'package:meta/meta.dart';
 
 import 'package:forui/forui.dart';
 
-/// A group of form fields that represent a single selection.
+/// A set of items that are treated as a single selection.
 ///
 /// Typically used to group multiple [FSelectGroupItem.checkbox]s.
 ///
@@ -61,7 +61,7 @@ class FSelectGroup<T> extends StatelessWidget {
       error: error,
       child: ListenableBuilder(
         listenable: controller,
-        builder: (context, child) => Column(
+        builder: (context, _) => Column(
           children: [
             for (final item in items)
               item.builder(

forui/lib/src/widgets/select_group/select_group_controller.dart

@@ -12,11 +12,11 @@ abstract class FSelectGroupController<T> with ChangeNotifier {
   void onChange(T value, bool selected);
 
   /// The values that are shallow copied.
-  Set<T> get values => {..._values};
-
-  /// Whether a value is selected.
+  /// Returns true if a value is selected.
   bool contains(T value) => _values.contains(value);
 
+  Set<T> get values => {..._values};
+
   @override
   bool operator ==(Object other) =>
       identical(this, other) ||
@@ -52,8 +52,9 @@ class FMultiSelectGroupController<T> extends FSelectGroupController<T> {
 
   /// Creates a [FMultiSelectGroupController].
   ///
-  /// The [min] and [max] values are the minimum and maximum number of selections allowed. The [min] value must be
-  /// greater than or equal to 0. If the [max] value is negative, there is no maximum.
+  /// The [min] and [max] values are the minimum and maximum number of selections allowed. Defaults to no minimum or maximum.
+  /// # Contract 
+  /// Throws [AssertionError] if [min] < 0 or [max] < [min].
   FMultiSelectGroupController({
     int min = 0,
     int max = -1,

forui/lib/src/widgets/select_group/select_group_item.dart

@@ -18,7 +18,7 @@ class FSelectGroupItem<T> with Diagnosticable {
     required this.builder,
   });
 
-  /// Creates a [FSelectGroupItem] with a [FCheckbox] as the builder.
+  /// Creates a [FSelectGroupItem] that wraps a [FCheckbox].
   factory FSelectGroupItem.checkbox({
     required T value,
     FCheckboxSelectGroupStyle? style,
@@ -32,7 +32,7 @@ class FSelectGroupItem<T> with Diagnosticable {
     ValueChanged<bool>? onFocusChange,
     Key? key,
   }) =>
-      FSelectGroupItem<T>(
+      FSelectGroupItem(
         value: value,
         builder: (context, onChange, selected) {
           final computedStyle = style ?? context.theme.selectGroupStyle.checkboxStyle;