Improve documentation of late boot unlocker

[dnoliver]

Hello!

Documentation in https://github.com/latchset/clevis/blob/3375b081f04f5105dfa654c15ac5c8c98756dd1b/src/luks/clevis-luks-unlockers.7.adoc#late-boot-unlocking is poor. It is required to browse crypttab documentation to get the end to end flow working, and it is not clear what to expect.

It would be nice if the documentation was more self contained. For example, adding the required example configuration for crypttab and fstab would be nice.

# requires clevis-systemd package
systemctl enable clevis-luks-askpass.path
# crypttab configuration
echo "encrypted_mapper /dev/mapper/encrypted_partition none _netdev" >> /etc/crypttab
# fstab configuration
echo "/dev/mapper/encrypted_mapper /path/to/mountpoint ext4 defaults,_netdev  0 2" >> /etc/fstab