[RFC] Doc update: Make Product getters and setters explicit

I am not using a Symfony form to update my entity but rather a custom application to do it. I was injecting a `Symfony\Component\HttpFoundation\File\File` into my `Product::setImage()` method and this bundle was silently failing in `\Vich\UploaderBundle\Storage\AbstractStorage` on line `50`:
```
if (is_null($file) || !($file instanceof UploadedFile)) {
    continue;
}
```

I need to instead inject an `UploadedFile` instance. This would be obvious using Symfony Form but perhaps not otherwise as not mentioned in the documentation. Providing these getters and setters in the docs makes it more obvious imo. Please let me know what you think and thanks for the bundle!