Include reference to database `_design/auth` ddoc

## Expected Behavior
Documentation should explain how `_design/auth` works on a database.

## Current Behavior
As far as I can tell, `_design/auth` isn't mentioned anywhere in our docs - a clear oversight.

```bash
$ cd src/docs/
$ grep -r _design/auth *
$
```

## Possible Solution
Should go in the Design Documents chapter since it's a special design document.