Although it was a long time ago, i think it may be worth to be noted here, just in case someone is interested. I work hard (at least, i hope ;)) to update the Serial Communications chapter of the FreeBSD Handbook in the past months, in particular the Introduction section. The work was heavily done with Gary W. Swearingen which take the PR for himself, helped by his mentor Giorgos Keramidas (not less), and the participation of Yar Tikhiy.
This work was done under the FreeBSD problem report docs/85355.
For reference only, here are most links to the different steps which drive us to the current committed code, as found in the documentation project source tree:
So, here it is: Serial Communications chapter.