docs: deprecated: add entry for "fluent-async-connect" log-opt
docs: deprecated: add entry for "fluent-async-connect" log-opt

This option was deperecated in the upstream fluentd logging driver v1.4.0,
and while we documented it as deprecated in the API changelog, there was
no mention yet in the deprecated docs.

relates to:

- https://github.com/fluent/fluent-logger-golang/pull/56
- https://github.com/moby/moby/pull/39086

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,docker/cli,cpuguy83/cli