Fixes to scope, make # announce correctly

as discssued in
https://github.com/twbs/bootstrap/issues/15117#issuecomment-63029993

- added ``scope="row"`` to row headers (can be either ``<td>`` or
``<th>``, AT doesn't care much)
- removed ``scope="col"`` where it was redundant
- make "#" announce correctly in Jaws with an ``<abbr>``
- simplified table examples with (to me) unnecessary
``rowspan``/``colspan`` (unless you really want complex tables, in which
case we would need full-on ``id`` and ``headers`` attributes to make
sure each table cell has an explicit association with the related header
cells - overkill, in my opinion)