Some of the labels on that picture are wrong. On the black plug, the bottom cavity labeled "ground" and the upper cavity labeled "spare ignition" are reversed. Also looks like the "ignition" and "starter" labels on the white plug are reversed as well.
The "ground during crank only" means that the ignition switch has a set of contacts inside that momentarily connect those terminals to ground when the switch is in the "start" position. They're used to provide a "bulb check" of the "temperature" and "brake" warning lights on the dash. Grounding those circuits causes those lights to momentarily turn on while cranking the engine so the driver knows the bulbs are good (not burned out).
Thanks to hatzie, you can download wiring diagrams and service manuals here -->
http://67-72chevytrucks.com/vboard/s...d.php?t=545416 Note: he doesn't have the 1981 wiring diagram available but the ignition switch wiring should be the same for 80 or 82.