Add the mention of `vendor/scoper-autoload.php` in usage docs

### Discussed in https://github.com/humbug/php-scoper/discussions/1101

<div type='discussions-op-text'>

<sup>Originally posted by **hirasso** January  3, 2025</sup>
Basically the title says it all. Should I require `vendor/autoload.php`, `vendor/scoper-autoload.php`, or both in my entry file? I couldn't find any documentation about this.</div>