Ready: Document dvv_enabled

Response to #1056.

- [x] Bucket type
- [x] Buckets.md
- [x] Add DVV theory doc

**PLEASE NOTE**: This PR should not be merged until PRs #999 and #1051 are merged, as it was written in anticipation of the changes made in those PRs.
@seancribbs - if you have time, could you provide a rewrite with correct information? Or, provide a little more information for @lucperkins or myself to rewrite, as we are not experts on the implementation of DVVs. Thanks.