[skip ci] Add comment for seek (#8811)

Signed-off-by: xige-16 <xi.ge@zilliz.com>

internal/msgstream/mq_msgstream.go

@@ -432,6 +432,7 @@ func (ms *mqMsgStream) Chan() <-chan *MsgPack {
 	return ms.receiveBuf
 }
 
+// Seek reset the subscription associated with this consumer to a specific position
 func (ms *mqMsgStream) Seek(msgPositions []*internalpb.MsgPosition) error {
 	for _, mp := range msgPositions {
 		consumer, ok := ms.consumers[mp.ChannelName]