DBDB (data-store) chapter for review!

I suggest ignoring the diff for ``chapter.md`` and just reading it directly. :)
Yeah, it's a tricky thing. There are two isolation-correct outcomes, both of which can be arrived at by running the transactions one after another (in some order). Isolation allows for concurrency as long as you get one of those outcomes. I reversed the halves of that sentence; maybe it's a bit easier to read this way?

>  …full isolation requires that even when running them concurrently, the result is the same as if they had been run one after another (in some order).