[skip ci]Fix golint error in memory_kv (#10861)

Signed-off-by: zhenshan.cao <zhenshan.cao@zilliz.com>

internal/kv/mem/mem_kv.go

@@ -19,6 +19,8 @@ import (
 	"github.com/google/btree"
 )
 
+// MemoryKV implements DataKV interface and relies on underling btree.BTree.
+// As its name implies, all data is stored in memory.
 type MemoryKV struct {
 	sync.RWMutex
 	tree *btree.BTree