Clean-up docs for the --web-renderer option (#70880)

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -463,11 +463,10 @@ abstract class FlutterCommand extends Command<void> {
     argParser.addOption('web-renderer',
       defaultsTo: 'html',
       allowed: <String>['auto', 'canvaskit', 'html'],
-      help: 'Which rendering backend to use for Flutter for Web.'
-          'auto      - Use the HTML renderer on mobile devices,'
-          '            and CanvasKit on desktop devices.'
-          'canvaskit - Always use the CanvasKit renderer.'
-          'html      - Default. Always use the HTML renderer.',
+      help: 'The renderer implementation to use when building for the web. Possible values are:\n'
+            'html - always use the HTML renderer. This renderer uses a combination of HTML, CSS, SVG, 2D Canvas, and WebGL. This is the default.\n'
+            'canvaskit - always use the CanvasKit renderer. This renderer uses WebGL and WebAssembly to render graphics.\n'
+            'auto - use the HTML renderer on mobile devices, and CanvasKit on desktop devices.',
     );
   }