Add explanatory comments to MapNotifier in optimistic update examples

- Add comment explaining widgets watching MapNotifier will automatically rebuild
- Applied to all three examples using MapNotifier:
  - optimistic_simple_delete_example.dart
  - optimistic_undoable_delete_example.dart
  - optimistic_undoable_toggle_example.dart
- Clarifies reactive nature of MapNotifier for readers

Author: escamoteur

code_samples/lib/command_it/optimistic_simple_delete_example.dart

@@ -4,6 +4,7 @@ import '_shared/stubs.dart';
 
 // #region example
 class TodoManager {
+  // MapNotifier is a reactive map - widgets watching it will automatically rebuild on any data change
   final todos = MapNotifier<String, Todo>();
 
   late final deleteCommand = Command.createAsyncNoResult<Todo>(

code_samples/lib/command_it/optimistic_undoable_delete_example.dart

@@ -4,6 +4,7 @@ import '_shared/stubs.dart';
 
 // #region example
 class TodoManager {
+  // MapNotifier is a reactive map - widgets watching it will automatically rebuild on any data change
   final todos = MapNotifier<String, Todo>();
 
   late final deleteTodoCommand = Command.createUndoableNoResult<Todo, Todo>(

code_samples/lib/command_it/optimistic_undoable_toggle_example.dart

@@ -4,6 +4,7 @@ import '_shared/stubs.dart';
 
 // #region example
 class TodoManager {
+  // MapNotifier is a reactive map - widgets watching it will automatically rebuild on any data change
   final todos = MapNotifier<String, Todo>();
 
   late final toggleCompleteCommand =