13.2R/ final countdown: corrections, improvements

Fix a handful of errors, some of which were noted in last year's bug
264113 for the /13.1R/ area but not fixed retrospectively.

Five commit message in underlying
https://github.com/freebsd/freebsd-doc/pull/165/commits

Not intended to be comprehensive, given the timescale;
https://reviews.freebsd.org/D39480#898747
draws attention to the absence of rtw88(4) from hardware notes, and
so on.

https://bugs.freebsd.org/264113

PR: 264113
Approved by: re (cperciva, gjb)
Pull request: https://github.com/freebsd/freebsd-doc/pull/165
Differential revision: https://reviews.freebsd.org/D39480