Initial fix to generalise use of modals. Generally, true role="dialog" containers would require only forms/controls as content, with all other text correctly and explicitly associated via aria-labelledby/aria-describedby and similar...but as authors use modals also as generic overlays, this extra role is required to get AT (particularly NVDA) out of "forms" mode and reenable reading keys.
Closes #15875 (closed)