x86-assembly: update the article a bit
Minor tweaks for consistency/readability. Actual content is not touched.
- Replace references to "chapter" with "article".
- Add a NOTE of the historical nature of the document.
- Clarify assemblers available on FreeBSD.
- List the trademarks used in the article, and reduce the (R) symbol usage. Per the Doc Primer, it only needs to be emitted with the first usage of the trademark.
- Add more tags for SEO
Reviewed by: 0mp, emaste (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45962