Ready: Document missing configs

There are some stray config parameters that show up in the output of `riak config effective` that need to be added.

Some have already been added. The following have not:

- [x] `leveldb.total_mem_percent`
- [x] `leveldb.block_cache_threshold`
- [x] `leveldb.compression`
- [x] `metadata_cache_size`
- [x] `object.merge_strategy` (actually it's not clear how this differs from `buckets.default.merge_strategy`...will investigate)
- [x] `fullsync_use_background_manager`

I also added section descriptions and did some general cleanup.