Improve text of [R7-8] and [R7-9] Closes #38

[jboehm1911]

This pull request makes two changes:

1. It removes the formulation "whitelist" from attribute referencing in [R7-8].
2. It updates attribute referencing to also allow references through attribute names, not only through attribute definitions (as before). This way, one can also reference for example to the URL attribute that is used in handles. Smaller textual updates are made in [R7-8] and [R7-9] for this.

[github-actions]

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