Serializer documentation can be improved (possibly contains an error?)

2 part question, both relating to (Symfony) Serializer docs.

1:
The Serializer docs show an example of decorating the json-ld normalizer to add a few fields, but wouldn't a completely custom serializer be better so the `getSupportedTypes` can be used to efficiently decide if this serializer should be used instead of having 20,30, maybe 100s of services decorating the json-ld normalizer and it having to go through all those layers with like `if (!$data instanceof ...)` or `if (!is_a($type, SomeClass::class, true))`

Should we add an example for a cache-able/scalable solution that doesn't involve decorating the json-ld one?

2:
Then looking at the example code here:
https://api-platform.com/docs/core/serialization/#changing-the-serialization-context-on-a-per-item-basis-for-symfony

Specifically:
```php
public function supportsNormalization($data, $format = null, array $context = [])
{
// Make sure we're not called twice
if (isset($context[self::ALREADY_CALLED])) {
return false;
}

return $data instanceof Book;
}

public function getSupportedTypes(?string $format): array
{
return [
Book::class => true
];
}
```

Returning `true` on `getSupportedTypes` means the usage of the serializer gets cached, and the `supportsNormalization` method is only checked **once**, then never again. So adding the `self::ALREADY_CALLED` in the `normalize` method doesn't do anything,... next time it goes through this normalizer, the `supportsNormalization` is skipped and boom, it now executed normalize on an item that has possibly already gone through it?

I can contribute a change for both things, but would like to get a second opinion first to see if I'm maybe missing something.