You're right, this should be a hint. Done in https://github.com/chrismedrela/django/commit/19326cb5e89822a2b341011fabb5b77b68de1742.

The main reason to have hint separated from error message is to improve errors by forcing developers to think carefully if they can provide any hint. If you cannot provide a sensible hint, you have to type:
```
Error('Error message', hint=None)
```

The second advantage is that we can print hints differently, but this is a minor advantage. Hints are printed in a new line, like this:
```
applabel.modellabel: Error message.
        HINT: Hint.
```