Add docs around multiple attach

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container.go

@@ -32,6 +32,9 @@ type Container interface {
 	//
 	// If IOAttach options are passed the client will reattach to the IO for the running
 	// task. If no task exists for the container a NotFound error is returned
+	//
+	// Clients must make sure that only one reader is attached to the task and consuming
+	// the output from the task's fifos
 	Task(context.Context, IOAttach) (Task, error)
 	// Image returns the image that the container is based on
 	Image(context.Context) (Image, error)