Description of problem: DNF system upgrade is a new feature which simplify upgrade process. However it would save times to just point an url to a wiki page documenting ALL the upgrade issues, like You SHOULD read http://... BEFORE to run dnf system-upgrade reboot For example, Postgresql upgrade is broken when extensions are used cf. https://bugzilla.redhat.com/show_bug.cgi?id=1055293, issue reported in 2014 ! It means actually Fedora's Postgresql is nearly useless ... !!! Thus Fedora should recommend to dump Postgresql databases before to launch dnf system-upgrade reboot. Note in some cases, these databases are for test purpose and not for production. Thus it make sense to upgrade without much cares. This information is not reported in https://fedoraproject.org/wiki/DNF_system_upgrade https://fedoraproject.org/wiki/Common_F26_bugs
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This bug appears to have been reported against 'rawhide' during the Fedora 29 development cycle. Changing version to '29'.
I believe that links that you mentioned (https://fedoraproject.org/wiki/DNF_system_upgrade, https://fedoraproject.org/wiki/Common_F26_bugs) is the best source for fedora users. We should not hardcode any url in dnf, because dnf is used not only by Fedora, but also other distros.
But you can provide a vendor hook (basically a file on disk or a network ressource) to display important information. However I understand it is not the job of a low level tool. It should be the job of the distribution to ensure users will not break their systems (as try to do Windows).