Changes proposed in this Pull Request

Adding a GlobalConstraint raised a warning for an unknown bus. This is because #1250 introduced the so far undocumented attribute bus.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.