Improve Documentation of the bits Parameter for the PartitionStrategies

[jbouffard]

Both SpatialPartitionStrategy and SpaceTimePartitionStrategy have the bits parameters whose role and importance aren't very clear from the documentation. The reason for this is that that parameter's role is somewhat complex and is difficult to explain in way that's easy to understand and concise. It would be good for both the users and us if we could come up with a better explanation for it.