Ready: Update backend-specific docs

- [x] Bitcask
  - [x] Disk reclamation settings from issue #1057 
- [x] Memory
 - [x] Ensure that `max_memory` is documented as a *per vnode* setting, not per node or per cluster
- [x] Multi

LevelDB changes are being made in a different branch/PR.
Should add `{io_mode, erlang},` for consistency with riak.conf