Update tenants.blade.php

Updates the Tenants topic to explain how to access the central context from a tenant's perspective.

source/docs/v3/tenants.blade.md

@@ -130,6 +130,23 @@ You may access the current tenant using the `tenant()` helper. You can also pass
 
 Alternatively, you may typehint the `Stancl\Tenancy\Contracts\Tenant` interface to inject the model using the service container.
 
+## Accessing the central application {#accessing-the-central-application}
+
+When your code executes in the context of a tenant (for instance, when the `DatabaseMigrated` event is dispatched), you may access the central application context by using the `tenancy()->central($callback)` function.
+
+For instance, if you want to retrieve the list of users of the central application when the code executes in a tenant's context, you may write:
+
+```php
+public function getCentralUsers()
+{
+    // here we are in the tenant's context. \App\Models\User::all() would return the current tenant's users.
+    return tenancy()->central(function () {
+        // here we are in the central app context
+        return \App\Models\User::all();
+    });
+}
+```
+
 ## Incrementing IDs {#incrementing-ids}
 
 By default, the migration uses `string` for the `id` column, and the model generates UUIDs when you don't supply an `id` during tenant creation.