fix golint issues in core/contextx (#477)

core/contextx/deadline.go

@@ -5,6 +5,8 @@ import (
 	"time"
 )
 
+// ShrinkDeadline returns a new Context with proper deadline base on the given ctx and timeout.
+// And returns a cancel function as well.
 func ShrinkDeadline(ctx context.Context, timeout time.Duration) (context.Context, func()) {
 	if deadline, ok := ctx.Deadline(); ok {
 		leftTime := time.Until(deadline)