[skip ci] Add comment for filterInvalidInsertMessage (#12734)

Signed-off-by: Nbigsheeper <yihao.dai@zilliz.com>

internal/querynode/flow_graph_filter_dm_node.go

@@ -165,6 +165,7 @@ func (fdmNode *filterDmNode) filterInvalidDeleteMessage(msg *msgstream.DeleteMsg
 	return msg
 }
 
+// filterInvalidInsertMessage would filter out invalid insert messages
 func (fdmNode *filterDmNode) filterInvalidInsertMessage(msg *msgstream.InsertMsg) *msgstream.InsertMsg {
 	sp, ctx := trace.StartSpanFromContext(msg.TraceCtx())
 	msg.SetTraceCtx(ctx)