Clarify shutdowner can only be used on started fx applications (#734)
Clarify shutdowner can only be used on started fx applications (#734)

* Clarify shutdowner can only be used on started fx applications

* Update shutdown.go

Co-authored-by: Abhinav Gupta <d1f85e55fba9f5b6db087e58762c7c8e370f6a57@uber.com>
uber-go/fx,uber-go/fx