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.
"loses a disk" is kinda specific. Maybe "For maximum safety, if an etcd member suffers any sort of data corruption, it must be removed from the cluster. It may be added back to the cluster once the source of the data corruption has been addressed."