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.
I think this section could be merged with the above. It's repetitive. Just add, after the previous paragraph:

"If you're using `app.config`, these values are named `frag_threshold`, `dead_bytes_threshold` and `small_file_threshold`, respectively."

Also this way, you can take advantage of the riakconf/appconfig tabs to reduce screen real estate.