Hello,
You use the following woraround:
@startuml
alt foo
Bob->Alice : hello
note over John_smith, An_other_actor: Some explication \non the large unused\n space beside the alt block.
Bob<-Alice : hello
end
note over John_smith, An_other_actor: Another expalanation
Alice->John_smith : hello
Alice->An_other_actor : hello
@enduml
It would be also nice to have the possibility to put negative space like this:
|| -10||
With this new feature, you will have exactly what you want
Hope this helps