Service update method does not match either docker-py nor docker Engine API spec

## Long story short

We are trying to update a service using the library. However, the library does not offer the same functionality as the official specifications and it is just offering to perform a rollback.

In order to update the service, we are now deleting and creating it again which leaves us the service lacking the previous spec.

Docker official specs:
[docker-py](https://github.com/docker/docker-py/blob/master/docker/api/service.py#L363)
[docker engine API](https://docs.docker.com/engine/api/v1.41/#operation/ServiceUpdate)

* Expected behaviour: service update method match docker-py spec or docker Engine API spec
* Actual behaviour: missing update service features (only rollback is allowed)

## How to reproduce

1. We first register a service by calling `docker_client.services.create(data)` where data matches the required arguments from [aiodocker](https://github.com/aio-libs/aiodocker/blob/master/aiodocker/services.py#L39)
2. When trying to update the service registered in the 1st step, we are not able to pass the new spec as a method parameter and we are just able to send the [rollback](https://github.com/aio-libs/aiodocker/blob/master/aiodocker/services.py#L39)

## Your environment

1. Python 3.7.6
2. Docker version 20.10.11, build dea9396
3. aiodocker==0.19.1