update action testing documentation to include testing modals

packages/actions/docs/09-testing.md

@@ -86,6 +86,25 @@ it('stops sending if invoice has no email address', function () {
 });
 ```
 
+## Testing modal content
+
+To assert against the content of a modal, you should first mount the action (rather than call it which resolves the modal). You can then use [livewire assertions](https://livewire.laravel.com/docs/testing#assertions) such as `assertSee` to assert the modal has rendered as you expect:
+
+```php
+use function Pest\Livewire\livewire;
+
+it('confirms the target address before sending', function () {
+    $invoice = Invoice::factory()->create();
+    $recipientEmail = $invoice->company->primaryContact->email;
+
+    livewire(EditInvoice::class, [
+        'invoice' => $invoice,
+    ])
+        ->mountAction('send')
+        ->assertSee($recipientEmail);
+});
+```
+
 ## Errors
 
 `assertHasNoActionErrors()` is used to assert that no validation errors occurred when submitting the action form.
@@ -126,7 +145,7 @@ it('can send invoices to the primary contact by default', function () {
         ])
         ->callMountedAction()
         ->assertHasNoActionErrors();
-        
+
     expect($invoice->refresh())
         ->isSent()->toBeTrue()
         ->recipient_email->toBe($recipientEmail);