Improve mock_constructor() old pointer error message

The current error message:

```
BaseException: Attribute getting after the class has been used with mock_constructor() is not supported! After using mock_constructor() you must get a pointer to the new mocked class...
```

Is hard to understand. Let's:

- Make this message clearer.
- Add links to the docs. This requires having a place where the version is defined so the doc link can be constructed correctly `https://testslide.readthedocs.io/en/$VERSION/mock_constructor/index.html`.