[8.x] Document new namespaced seeders and factories

[driesvints]

Because the make:seeder & make:factory command generate namespaced classes from their stubs, the upgrade to implement these namespaces is a required one. Therefor I think it's good that we document this in the Laravel 8 upgrade guide.

Closes laravel/framework#34243

[browner12]

can we accomplish the autoloading in one line?

 "autoload": {
        "psr-4": {
            "App\\": "app/",
            "Database\\": "database/"
        }
    },

[vladislavtkachenko]

browner12, no, PSR-4 error

[browner12]

we would just need to capitalize the folders within /database, correct?

[driesvints]

@browner12 no we can't as schema and migrations aren't namespaced.

[browner12]

thanks!