docs(ct): document class instance serialization limitation

docs/src/test-components-js.md

@@ -174,7 +174,7 @@ When Playwright Test is used to test web components, tests run in Node.js, while
 
 This however, is introducing a number of limitations:
 
-- You can't pass complex live objects to your component. Only plain JavaScript objects and built-in types like strings, numbers, dates etc. can be passed.
+- You can't pass complex live objects to your component. Only plain JavaScript objects and built-in types like strings, numbers, dates etc. can be passed. Class instances will lose their prototype methods during serialization — use plain objects or [test stories](#test-stories) instead.
 
 ```js
 test('this will work', async ({ mount }) => {
@@ -184,6 +184,11 @@ test('this will work', async ({ mount }) => {
 test('this will not work', async ({ mount }) => {
   // `process` is a Node object, we can't pass it to the browser and expect it to work.
   const component = await mount(<ProcessViewer process={process}/>);
+
+  // Class instances lose their prototype methods when serialized.
+  // Calling `user.greet()` in the component will fail.
+  class UserModel { greet() { return `Hello, ${this.name}`; } }
+  await mount(<UserCard user={new UserModel()}/>);
 });
 ```