Add persistence docs + adjustments

- Increase the level of docs around why the persistence
  objects exist and what they are used for and how an
  engine interacts with persistence objects to accomplish
  the wider goals.
- Pass backend conf as arg, not kwarg and to match with the
  other fetch/load functions allow for providing a kwargs
  which can be used for backend specific params.

Change-Id: Ia83e63196bcfcc966f68212f84f79a1e9f18e8bc
Merge "Add persistence docs + adjustments"