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.
This issue complicates this paragraph: #960 

Basically, `o_sync` *is* supported when using `nif` `io_mode`, and not supported in `erlang` `io_mode`.

If you enable `nif` `io_mode` and `o_sync`, you will still get this incorrect warning:

```
 [warning] <0.445.0>@riak_kv_bitcask_backend:check_fcntl:429 {sync_strategy,o_sync} not implemented on Linux
```

To disable the check that generates this warning, modify the `riak_kv` section of the `app.config` file in this manner:

```
 {riak_kv, [
            {o_sync_warning_logged, false},
```