Add group chat MessageReceive documentation

This fixes the issue in the group chat message processor. The crux of the issue is that we were returning to front end via the callback a JSON marshalled Go structure, and were not making this clear through documentation.

type MessageReceive struct {
	GroupID   *id.ID
	ID        group.MessageID
	Payload   []byte
	SenderID  *id.ID
	Timestamp time.Time
}

This MR creates a bindings layer equivalent and returns a JSON marshalled version of that through the callback, and attempts to make it clear by updating documentation.