Be more explicit about what the {{USER_ID}} metadata field is for

Justin says:

I think being more explicit and adding a comment would help a lot.  Something like this:

```
$user_id = ''; // Fetch from your database

$verification_session = $stripe->identity->verificationSessions->create([
  'type' => 'document',
  'metadata' => [
    'user_id' => $user_id,
  ]
]);
```