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.
Actually this whole paragraph is misleading... version vectors/vector clocks do the exact things that DVVs claim to do in this paragraph, but DVVs *also* associate the event where a sibling is written with the sibling (the "dot"). That last bit is missing from the explanation.