[RFC] Doc update: Make Product getters and setters are 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 to inject and `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, thanks for the bundle!