__vars__ in documentation

### Search for existing feature requests

- [X] I have searched the issue tracker and confirmed that I was unable to find existing issues for the feature I am requesting.

### The feature request

Please provide some pointers in the documentation and/or the config variables page about how variables can be used in code.

### Proposed solution

```python
print(__vars__['var_name'])
```

### Additional context

_No response_