[skip ci] Add comment for queryLock (#11376)

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

internal/querynode/collection_replica.go

@@ -131,6 +131,7 @@ type collectionReplica struct {
 	etcdKV *etcdkv.EtcdKV
 }
 
+// queryLock guards query and delete operations
 func (colReplica *collectionReplica) queryLock() {
 	colReplica.queryMu.Lock()
 }