13.2 relnotes: update hostid/machine-id items

The details of the addition of /etc/machine-id have changed, along
with the type of /etc/hostid if it is generated. Update the release
notes accordingly. Also document uuidgen -r, used by the hostid
script. Also include a change from my previous review that I
missed.

Reviewed by: carlavilla, tijl
Differential Revision: https://reviews.freebsd.org/D39101
Approved by: carlavilla