fix golint issues in core/syncx (#526)

core/syncx/donechan.go

@@ -6,23 +6,27 @@ import (
 	"github.com/tal-tech/go-zero/core/lang"
 )
 
+// A DoneChan is used as a channel that can be closed multiple times and wait for done.
 type DoneChan struct {
 	done chan lang.PlaceholderType
 	once sync.Once
 }
 
+// NewDoneChan returns a DoneChan.
 func NewDoneChan() *DoneChan {
 	return &DoneChan{
 		done: make(chan lang.PlaceholderType),
 	}
 }
 
+// Close closes dc, it's safe to close more than once.
 func (dc *DoneChan) Close() {
 	dc.once.Do(func() {
 		close(dc.done)
 	})
 }
 
+// Done returns a channel that can be notified on dc closed.
 func (dc *DoneChan) Done() chan lang.PlaceholderType {
 	return dc.done
 }