docs: add example for the `prune` command
Giving at least one example helps clear up the syntax, especially since most options will follow a very similar pattern.
This instance will be upgraded to Heptapod 0.31.0 (final) on 2022-05-24 at 14:00 UTC+2 (a few minutes of down time)
Giving at least one example helps clear up the syntax, especially since most options will follow a very similar pattern.