FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)
I think it's only useful when resolving conflicts, so we should lead with that. Something like 

```
When resolving conflicts, this method can be used to delete any non-deleted 
leaf revision of a document. {@see resolveConflictsForDocument} handles this 
deletion step during conflict resolution, so it's not usually necessary to call this 
method in this way from client code. See the doc/conflicts.md document for
more details.
```