kvcoord: add a log msg

Release note: None
Release justification: low risk, just logging

pkg/kv/kvclient/kvcoord/txn_interceptor_committer.go

@@ -208,6 +208,7 @@ func (tc *txnCommitter) SendLocked(
 	// could be a combination of protos from responses, all merged by
 	// DistSender.
 	if pErr := needTxnRetryAfterStaging(br); pErr != nil {
+		log.VEventf(ctx, 2, "parallel commit failed since some writes were pushed")
 		return nil, pErr
 	}