Improve viewset action docs

[k0t3n]

Hi,
I encountered a problem when overriding viewset-level attributes doesn't work for actions if a route is configured as .as_view().

Example:

# views.py
class TestView(viewsets.ModelViewSet):
    @action(methods=['get'], detail=False, url_name='test_action', permission_classes=[IsAuthenticated,])
    def test_action(self, request):
        return Response()

# urls.py
urlpatterns = [
    path('test_urlpattern/', TestViewSet.as_view({'get': 'test_action'},)),
]

# tests.py
@pytest.mark.django_db
def test_case(client):
    url = reverse('test_urlpattern')
    response = client.get(url)
    assert response.status_code == 200  # expected 401!

You can override it using initkwargs only

# urls.py
urlpatterns = [
    # permission will be added
    path('test_urlpattern/', TestViewSet.as_view({'get': 'test_action'}, permission_classes=[IsAuthenticated, ])),
]

According to the docs, it's not clear this feature works for Router configuration only. No exceptions, warnings, or even related github issues were found regarding this.