docs: included doc comments

lib/game/components/initial_position.dart

@@ -1,6 +1,9 @@
 import 'package:flame_forge2d/flame_forge2d.dart';
 
+/// Forces a given [BodyComponent] to position their [body] to an
+/// [initialPosition].
 mixin InitialPosition on BodyComponent {
+  /// The initial position of the [body].
   late final Vector2 initialPosition;
 
   @override