u3g.4: Fall cleaning

Many people come on our social spaces asking about router projects,
can't find info about supported cellular modems, and say they're
sticking to $other_OS. Thanks to the fantastic talk by Ian from
Metify, I now know that is not true, we have them and they work great.

Therefore, tell everyone else by massaging this page for
discoverability in the manual and the release hardware notes.

+ Document description: s/datacards/cellular modems/ for apropos search
+ HARDWARE: s/adapters/cellular modems/ for release hardware notes
+ move things which shouldn't be in Rel HW notes to description
+ massage things around for readability and flow + tag spdx
+ xref supported connections

MFC after: 3 days
Reported by: allanjude (u3g can also provide GPS port)
Reported by: Jared Trout <jaredtrout@google.com> (cellular in title)
Reported by: emaste (specify mpd, editorial style instruction)
Reported by: Metify (modem drivers exist)
Reviewed by: imp, emaste, allanjude
Pull Request: https://github.com/freebsd/freebsd-src/pull/1519

(cherry picked from commit 53484f516ac4ce3bf57a8c52c094ae67d79312d5)