docs: do not use definition-list for sub-topics

docs: do not use definition-list for sub-topics

The dl-tag isn't a neat tool for defining sub-headings, it's a semantic
tool for defining definitions and their meaning. Let's insetad use
normal sub-headings instead.

To make the last few paragraphs stand out from the above, let's add a
sub-heading for those as well.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3443>