OSQP solvers with fun set and no init_params have misleading error message

The error message for setting `fun` to a function and not providing `init_params` to a class such as `jaxopt.BoxOSQP` is incorrect:

```
ValueError: init_x must be provided when fun is not None.
```

There is no `init_x` parameter to the `run()` method. Also, there are no examples in the documentation of those functions for what should be in a `KKTSolution` object, and that object appears to be undocumented.