You can use this syntax to apply doc comments to the item they are in. So in this case this would be applied to the send_confirmation module. This is useful for module-level documentation.

This should probably be moved to be above MessageSentStatus.
You could also move the variant specific comments above the variant definition e.g.

enum MessageSentStatus {
  /// Message with id was processed by Tunnelbroker
  Success(String)
  ...
}

Shouldn't this be SendConfirmation?

Nit: "confirmation" sounds like the message already succeeded but this also contains message send failures (not entirely happy with my name suggestion too tho)

D9521