Comment: add some lines back

Signed-off-by: Hu Keping <hukeping@huawei.com>

daemon/graphdriver/driver.go

@@ -48,6 +48,7 @@ type ProtoDriver interface {
 	// String returns a string representation of this driver.
 	String() string
 	// Create creates a new, empty, filesystem layer with the
+	// specified id and parent. Parent may be "".
 	Create(id, parent string) error
 	// Remove attempts to remove the filesystem layer with this id.
 	Remove(id string) error