Clarify attribute mapping tables

[scottaohara]

for attributes which do have an impact on the a11y api, but do not have a perfectly matching aria attribute counterpart, they are listed as "not mapped".

But in the elements table, elements which have a11y mappings but no ARIA role are listed as "no corresponding wai-aria role".

The attribute table should follow this wording.

Both the element and attributes table should also add text in their preamble that states identifies what "no corresponding WAI-ARIA [ role/attr ]" means, since the spec identifies what everything else means.

[pkra]

@scottaohara is this something for other AAMs, i.e., should we talk about this on the editors call?

[scottaohara]

Potentially. Might as well talk about it there.