man: provide a basic guide to the `systemctl status` output (#4950)

 - Show example of all `systemctl status` output and documents what possible
   "Loaded:", "Active" and "Enabled" values mean.

 - Documents what different colors of the dot mean.

 - Documents "gotcha" with load-on-demand behavior which will report units as
   "loaded" even if they are only loaded to show their status.
   (From @poettering: https://github.com/systemd/systemd/issues/5063#issuecomment-272115024 )