When we configure an output channel for the handler as a `FluxMessageChannel`,
in most cases we assume an async processing for the reply.
Especially this is critical when reply is a reactive type, so in the async
mode it is "flattened" by the mentioned `FluxMessageChannel`.
Therefore, it is a bit awkward to require to set async explicitly,
when we have already configured output channel as a `FluxMessageChannel`
* Remove redundant config for async from `R2dbcDslTests`
* Mention the change in the docs
b999ac109c