doc: clarify the doc

I can never remember this so I think it could be added to the doc.

OT question:
I needed to do something like
```
    expect(()=> throw new InvalidArgument()).toThrowWith(where: (e) {
      expect(e is ParentType).toBeTrue()
    });
```

but I had written

```
    expect(()=> throw new InvalidArgument()).toThrowWith(where: (e) => e is ParentType);
```

Which does not work - or does not fail ;)

I think we could amend the lib to make both work (ie fail if the function returns false). What do you think ?