In the new Actvity Diagram syntax only conditional processing has an option to place labels on transitions.
It would be nice to have the option to add labels to any transition.
Ex:
:(transition label) "State"
(--------)
Actually, there is an undocumented feature here. You can try :
@startuml start :foo1; -> this is a label; :foo2; @enduml
We should document this some day...