Documentation: fixup the admin_guide

- Provide more concrete examples and explanation
- Cleanup the formatting to one sentence per line, this makes reviewing
  easier
- Point to existing docs on wal and snap instead of trying to duplicate
  it here again.
They are different thing. Append-only means the actual file is append-only, so you can safely read the file. Write-ahead means etcd always writes the data to the log file before it actually commit it, so you can always do full recovery from log.