Add cross-table headers attribute example

[WilcoFiers]

This update is based on feedback received from AG on the June 7th call.

Need for Call for Review: 1 week

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How to Review And Approve

• Go to the “Files changed” tab
• Here you will have the option to leave comments on different lines.
• Once the review is completed, find the “Review changes” button in the top right, select “Approve” (if you are really confident in the rule) or "Request changes" and click “Submit review”.
• Make sure to also review the proposed Call for Review period. In case of disagreement, the longer period wins.

[daniel-montalvo]

We should at some point update the example descriptions. Instead of "the xxx element" we probably want to have "this/these xx element(s)"

But that's a minor thing really.