fix golint issues in core/syncx (#526)

core/syncx/barrier.go

@@ -2,10 +2,12 @@ package syncx
 
 import "sync"
 
+// A Barrier is used to facility the barrier on a resource.
 type Barrier struct {
 	lock sync.Mutex
 }
 
+// Guard guards the given fn on the resource.
 func (b *Barrier) Guard(fn func()) {
 	b.lock.Lock()
 	defer b.lock.Unlock()