add information about backfilling serially

Why is this change being made?

I ran into a problem with backfilling some data that had already been backfilled because it ran DAG runs concurrently. This change is to lay out the process for dealing with similar situations in the future.

Does this MR meet the acceptance criteria?

Yes.

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what