docs: Update the dartdocs for FixedResolutionViewport (#3132)

Updates the dartdocs of `FixedResolutionViewport` since it has an
accidental copy paste from another viewport.

packages/flame/lib/src/camera/viewports/fixed_resolution_viewport.dart

@@ -4,9 +4,11 @@ import 'package:flame/camera.dart';
 import 'package:flame/extensions.dart';
 import 'package:flame/src/effects/provider_interfaces.dart';
 
-/// [FixedAspectRatioViewport] is a rectangular viewport which auto-expands to
-/// take as much space as possible within the canvas, while maintaining a fixed
-/// aspect ratio.
+/// The [FixedResolutionViewport] is a rectangular viewport which auto-expands
+/// to take as much space as possible within the canvas, while maintaining a
+/// fixed resolution and aspect ratio. I.e. the viewport will always show the
+/// same dimensions ([resolution]) of the game world, regardless of the size of
+/// the canvas.
 ///
 /// This viewport will automatically adjust its size and position when the
 /// game canvas changes in size. At the same time, manually changing the size