bectl.8: update /home

The default location for home directories is moving from /usr/home
to /home, including the default zfs datasets. Update accordingly.
Add zroot/usr/src as replacement example of nested datasets.

While here, mark zroot/var as "canmount off" as per current setup.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D40206