Fixes to scope

As discussed 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
- 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)