This is very documented, but there is a keyword "collections"
@startuml
skinparam sequenceParticipant underline
participant sender
collections collection
sender -> collection
@enduml
http://www.plantuml.com/plantuml/uml/JSsn3O0m34JHFbDu0LP06O4LBEF2md52xEnF40NfJkyNbpsmnoYQ_19hsB60yptOY8zPXAIX1GmxkQiOfxQeyyk9gYfJI3M7VwUlmhQlc2VEkmS0