Update headers attribute refer to data cells ensuring ancestor table has a role of table grid or treegrid

[giacomo-petri]

This pull request addresses the issue raised by @WilcoFiers, ensuring that headers attributes specified on a cell refers to cells in the same table element, when the table element has a role of table, grid or treegrid:

• added specific point about role in applicability;
• added specific inapplicable example.

Closes issue(s):

• closes Headers attribute rule should skip non-table roles [a25f45] #1928

Need for Call for Review:
This will require a 2 week Call for Review

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[carlosapaduarte]

@giacomo-petri we're updating the applicability of the rule, therefore the call for review should be for 2 weeks

[WilcoFiers]

Couple small formatting tweaks. Rest looks good.

[Jym77]

(test fails due to the missing [semantic role]: #semantic-role 'Definition of semantic role' in the references list)

As the changes were done