Add comment to explain the txn proposal race condition.

dgraph/cmd/zero/oracle.go

@@ -297,9 +297,11 @@ func (s *Server) commit(ctx context.Context, src *api.TxnContext) error {
 	}
 	// Propose txn should be used to set watermark as done.
 	err = s.proposeTxn(ctx, src)
-	// TODO: Ideally proposal should throw error if it was already aborted due to race.
 	// There might be race between this proposal trying to commit and predicate
-	// move aborting it.
+	// move aborting it. A predicate move, triggered by Zero, would abort all pending transactions.
+	// At the same time, a client which has already done mutations, can proceed to commit it. A race
+	// condition can happen here, with both proposing their respective states, only one can succeed
+	// after the proposal is done. So, check again to see the fate of the transaction here.
 	if s.orc.aborted(src.StartTs) {
 		src.Aborted = true
 	}