Fix `MultiChildLayoutDelegate.hasChild` doc (#126433)

fixes https://github.com/flutter/flutter/issues/115898

`hasChild` checks `_idToChild` map.  `_idToChild` is not assigned until `_callPerformLayout` is called.
So `hasChild` shouldn't be called in `getSize`.

https://github.com/flutter/flutter/blob/df789c9e76098e82f80f2c5de8b5560f360afa40/packages/flutter/lib/src/rendering/custom_layout.dart#L400-L404

Updated docs and example class names.

examples/api/lib/widgets/basic/custom_multi_child_layout.0.dart

@@ -6,10 +6,10 @@ import 'package:flutter/material.dart';
 
 /// Flutter code sample for [CustomMultiChildLayout].
 
-void main() => runApp(const ExampleApp());
+void main() => runApp(const CustomMultiChildLayoutApp());
 
-class ExampleApp extends StatelessWidget {
-  const ExampleApp({super.key});
+class CustomMultiChildLayoutApp extends StatelessWidget {
+  const CustomMultiChildLayoutApp({super.key});
 
   @override
   Widget build(BuildContext context) {
@@ -19,7 +19,7 @@ class ExampleApp extends StatelessWidget {
         // see the layout change.
         textDirection: TextDirection.ltr,
         child: Scaffold(
-          body: ExampleWidget(),
+          body: CustomMultiChildLayoutExample(),
         ),
       ),
     );
@@ -82,8 +82,8 @@ class _CascadeLayoutDelegate extends MultiChildLayoutDelegate {
   }
 }
 
-class ExampleWidget extends StatelessWidget {
-  const ExampleWidget({super.key});
+class CustomMultiChildLayoutExample extends StatelessWidget {
+  const CustomMultiChildLayoutExample({super.key});
 
   static const Map<String, Color> _colors = <String, Color>{
     'Red': Colors.red,

examples/api/test/widgets/basic/custom_multi_child_layout.0_test.dart

@@ -12,7 +12,7 @@ void main() {
     await tester.pumpWidget(
       const MaterialApp(
         home: Scaffold(
-          body: example.ExampleWidget(),
+          body: example.CustomMultiChildLayoutApp(),
         ),
       ),
     );
@@ -24,7 +24,7 @@ void main() {
     await tester.pumpWidget(
       const MaterialApp(
         home: Scaffold(
-          body: example.ExampleWidget(),
+          body: example.CustomMultiChildLayoutExample(),
         ),
       ),
     );
@@ -40,7 +40,7 @@ void main() {
     await tester.pumpWidget(
       const MaterialApp(
         home: Scaffold(
-          body: example.ExampleWidget(),
+          body: example.CustomMultiChildLayoutExample(),
         ),
       ),
     );

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -127,9 +127,11 @@ abstract class MultiChildLayoutDelegate {
 
   /// True if a non-null LayoutChild was provided for the specified id.
   ///
-  /// Call this from the [performLayout] or [getSize] methods to
-  /// determine which children are available, if the child list might
-  /// vary.
+  /// Call this from the [performLayout] method to determine which children
+  /// are available, if the child list might vary.
+  ///
+  /// This method cannot be called from [getSize] as the size is not allowed
+  /// to depend on the children.
   bool hasChild(Object childId) => _idToChild![childId] != null;
 
   /// Ask the child to update its layout within the limits specified by