…s (HypeMC)

This PR was merged into the 5.4 branch.

Discussion
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[HttpCache] Explain how to extend the `HttpCache` class

Followup to #16402.

In versions of Symfony prior to 5.4 the caching kernel was enabled by wrapping the default kernel into it inside the front controller, eg:

```php
// public/index.php
use App\CacheKernel;
use App\Kernel;

// ...
$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG']);
// Wrap the default Kernel with the CacheKernel one in 'prod' environment
if ('prod' === $kernel->getEnvironment()) {
    $kernel = new CacheKernel($kernel);
}
```

Starting from version 5.4 this was changed to be done through the configuration:

```yaml
# config/packages/framework.yaml
when@prod:
    framework:
        http_cache: true
```

However, it was never explained how the caching kernel can be extended in this case. Decorating the `http_cache` service seems like the best option.

Commits
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34809dd [HttpCache] Explain how to extend the HttpCache class