doc: re-wrap design-oob to 72 chars

I started with just adding some :term:`SoW` and similar to design-oob,
but then I realised this was 80-chars wrapped, not 72-chars. So I went
and re-wrapped most of it, plus adding the glossary references.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>