[skip ci] Add comment for getCollectionNum (#10915)

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

internal/querynode/collection_replica.go

@@ -53,6 +53,7 @@ type ReplicaInterface interface {
 	getCollectionByID(collectionID UniqueID) (*Collection, error)
 	// hasCollection checks if collectionReplica has the collection which id is collectionID
 	hasCollection(collectionID UniqueID) bool
+	// getCollectionNum returns num of collections in collectionReplica
 	getCollectionNum() int
 	getPartitionIDs(collectionID UniqueID) ([]UniqueID, error)
 	getVecFieldIDsByCollectionID(collectionID UniqueID) ([]FieldID, error)