mention `rules` in documentation where variables can be used

What does this MR do?

Variables can be used in rules:if too, the documentation was missing on this page.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it is only a documentation fix.
• Documentation (if required)
• Code review guidelines
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• Database guides
• Separation of EE specific content