Improve documentation on the Environment and Tenancy facade

[bkintanar]

Moved from tenancy/multi-tenant#273

$tenancy = app(Hyn\Tenancy\Environment::class);

// get website
$tenancy->website();
// get hostname
$tenancy->hostname();
// get customer (if any)
$tenancy->customer();
// set hostname
$hostname = Hostname::firstOrFail();
$tenancy->hostname($hostname);

[luceos]

Include the facade.