fix Model docstring

[alekracicot]

What is this PR about? ... Description: Change Model docstring Issue name Model docstring is outdated Issue number: #6040

Checklist

• [x ] Explain important implementation details 👆
• [x ] Make sure that the pre-commit linting/style checks pass. (really just changed the docstring.
• [ x] Link relevant issues (preferably in nice commit messages)
• [x ] Are the changes covered by tests and docstrings? No test needed, only doc)
• [ x] Fill out the short summary sections 👇

Major / Breaking Changes

• ...

Bugfixes / New features

• ...

Docs / Maintenance

• Change docstring for the class Model, replacing var by register_rv