[skip ci] Add comment for historical close (#13373)

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

internal/querynode/historical.go

@@ -68,6 +68,7 @@ func (h *historical) start() {
 	go h.watchGlobalSegmentMeta()
 }
 
+// close would release all resources in historical
 func (h *historical) close() {
 	// free collectionReplica
 	h.replica.freeAll()