Clarify temporal semantics for RunShutdownable.Shutdown

actor.go

@@ -99,9 +99,8 @@ type RunShutdownable interface {
 	// non-nil error explaining why. Shutdown does not need to be called in
 	// the latter case.
 	Run() error
-	// Shutdown shuts down the actor. It must be called while Run is
-	// running, and it must unblock Run even if it fails, the context
-	// expires, it returns an error, etc. Shutdown must return when or
+	// Shutdown shuts down the actor. It must unblock Run in all cases even
+	// on failure, context expiration, etc. Shutdown must return when or
 	// before the context expires, and must return ctx.Err if there is no
 	// other error to be returned. If Shutdown returns any error, the object
 	// must be treated as invalid and any other process which depends on it