Add support for displaying ordering fields

[Headmaster11]

class SomeViewSet(mixins.ListModelMixin, GenericViewSet):
    queryset = SomeModel.objects.all()
    filter_backends = (filters.OrderingFilter,)
    ordering_fields = ("field1", "field2")

    @extend_schema(
        parameters=[
            OpenApiParameter(
                name="ordering",
                description=f'Supports sorting by {" ".join(str(x) for x in ordering_fields)}. Use - to filter backwards',
            )
        ]
    )
    def list(self, request, *args, **kwargs):
        return super().list(request, *args, **kwargs)

without extend schema decorator, description for ordering is Which field to use when ordering the results. which is not very informative. Maybe it's better to have list of supported fields?

[tfranzel]

Hi @Headmaster11, that is a good point. The default actually comes out of DRF:

https://github.com/encode/django-rest-framework/blob/71e6c30034a1dd35a39ca74f86c371713e762c79/rest_framework/filters.py#L322

So we may either fix it upstream or introduce an override (OpenApiFilterExtension) on our side.