(--------)
This is not perfect, but you can have:
@startuml class a class b a --> b : ok note on link : some note @enduml
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuKhEIImkLaY4qabSYGgwkdOAIGfM2ldPN7dv9QaA-NaAEPbvsK2HulpSL0MG69T3QbuAK3S0