chore(restore): add docs for mutations in between incremental restore

If we do mutations in between incremental restores, those will
get lost. The next incremental restore will overwrite any data
that is written in between two incremental restores.

dgraphtest/local_cluster.go

@@ -73,7 +73,7 @@ func (u UpgradeStrategy) String() string {
 	case BackupRestore:
 		return "backup-restore"
 	case InPlace:
-		return "stop-start"
+		return "in-place"
 	case ExportImport:
 		return "export-import"
 	default:

systest/integration2/incremental_restore_test.go

@@ -87,5 +87,12 @@ func TestIncrementalRestore(t *testing.T) {
 			sort.Ints(data.Q[0].Money)
 			require.Equal(t, uids[j-1:i], data.Q[0].Money)
 		}
+
+		// Even when we do an in between mutations, it makes no difference.
+		// Incremental restore overwrites any data written in between.
+		if i == 10 {
+			_, err := gc.Mutate(&api.Mutation{SetNquads: []byte(`<10> <money> "4" .`), CommitNow: true})
+			require.NoError(t, err)
+		}
 	}
 }

worker/online_restore.go

@@ -371,6 +371,10 @@ func handleRestoreProposal(ctx context.Context, req *pb.RestoreRequest, pidx uin
 		if err := pstore.DropPrefix(dropAttrs...); err != nil {
 			return errors.Wrap(err, "failed to reduce incremental restore map")
 		}
+		// If there are any writes done after last incremental restore on badger DB,
+		// the restore or incremental restore will lose that data. This happens because
+		// the timestamp of the in between mutations etc will be lower than the timestamp
+		// of this incremental restore that we may be doing now (if it is one).
 		if err := sw.PrepareIncremental(); err != nil {
 			return errors.Wrapf(err, "while preparing DB")
 		}