[skip ci] Add comment for getPartitionNum (#11099)

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

internal/querynode/collection_replica.go

@@ -71,6 +71,7 @@ type ReplicaInterface interface {
 	getPartitionByID(partitionID UniqueID) (*Partition, error)
 	// hasPartition returns true if collectionReplica has the partition, false otherwise
 	hasPartition(partitionID UniqueID) bool
+	// getPartitionNum returns num of partitions
 	getPartitionNum() int
 	getSegmentIDs(partitionID UniqueID) ([]UniqueID, error)
 	getSegmentIDsByVChannel(partitionID UniqueID, vChannel Channel) ([]UniqueID, error)