man: explain ConditionNeedsUpdate a bit more
    
    We were effectively doing all post-upgrade scripts twice in Fedora. We got this
    wrong, so it's likely other people will get it wrong too. So let's explain
    what is actually needed to make this work, but also when it's not useful.
    
    (cherry picked from commit f75420a43a4dc3c69210f887b38f5df23660ea16)