doc: expand on libseat via ssh

Give a little more details about how running Weston via ssh or serial
terminal is best done, now that launcher-direct and weston-launch are
gone.

Hopefully the removal of launcher-direct also makes less people run
Weston as root, when seatd is the privileged process. Running 'weston'
as root might still work through libseat's builtin backend without
seatd.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>