Updated doc block for $media->model_id

[viirre]

The change made in https://github.com/spatie/laravel-medialibrary/pull/3341/files#diff-8adff69d13774eb09fba1b9b993d0be0d466cf8d43523c9d8050884207cd5901R38 breaks our Larastan checks.

We use strict comparison checks between the $media->model_id and an integer (eg: $media->model_id === $model->id) and since the doc block in the above PR says it's a string only, it breaks. But that column can of course also be an integer, so I guess that this was just missed in the PR.

This PR updates the doc block to be @property string|int instead.

[freekmurze]

Thanks!

[francoism90]

@viirre Instead of doing this by model_id, you can use the following:

$media = Media::first();

$media->model()->is($model);

See https://laravel.com/docs/10.x/eloquent#comparing-models