Hello A.,
You can use floating note, here is a proposal:
@startuml
interface HostSession{
+spawn_removed(): void
+child_added(): void
+child_removed(): void
+process_crashed(): void
+output(): void
+uninjected(): void
}
note "These 3 functions are implemented\nin LinuxHostSeesion." as N1
N1 -> HostSession::process_crashed
N1 -> HostSession::output
N1 -> HostSession::uninjected
@enduml

[See on PlantUML server]
See also documentation here:
If that can help,
Regards,
Th.