Editorial: Update note markup/markdown and styling

Using the `note: ...` markdown, Bikeshed produces nicely styled notes
with a "NOTE:" prefix automagically generated. This source was not
using that, so not getting the nice prefix. Fix that!

* `<aside>` was used for notes, examples, and advisements, but it's
  not a block element. Just use `<div>`.

* For notes, use `NOTE:` markdown where possible (single paragraphs)

* Otherwise, add "NOTE:" manually as a span, class=marker

* For a handful of cases where a note follows an algorithm and
  references variables within the algorithm, scooch the note inside
  the algorithm's containing div so that it is autolinked.

* Remove a stray unbalanced `</div>`

See speced/bikeshed#2562 for context.