Cause: The `oadm prune images` used to print confusing errors (e.g. operation timeout). And several of its options were not documented. Also the help was misleading (e.g. --registry-url flag is necessary only when passed with --confirm).
Consequence: User did not know what to do when timeout occured. User wasn't able to find out the solution even in the documentation.
Fix: Errors are now printed with hint. Documentation has been updated, help has been amended.
Result: N00b should be able to prune images now. Even outside of OpenShift cluster.