Document various systemd states

It's unclear what the directories do.

systemd source code tells us:

/* Get operational state from ifindex.
 * Possible states:
 *   off: the device is powered down
 *   no-carrier: the device is powered up, but it does not yet have a carrier
 *   dormant: the device has a carrier, but is not yet ready for normal traffic
 *   carrier: the link has a carrier
 *   degraded: the link has carrier and addresses valid on the local link configured
 *   routable: the link has carrier and routable address configured

We don't have dirs for degraded and carrier. Not sure if they get exposed or not.

We should have that in the manual page somewhere I guess, though systemd-networkd should probably document it itself too (https://github.com/systemd/systemd/issues/575)

Ubuntu bug: https://bugs.launchpad.net/ubuntu/+source/networkd-dispatcher/+bug/1787495