Add corrective action to help text when an app does not have a translation domain

Add pointer to KLocalizedString::setApplicationDomain in error message when an app has an empty domain.

This error message is now very long, but in my opinion much better because an uninformed user (me) now has enough information to go resolve the issue.

Doc link for anyone who happens to find this PR first: https://api.kde.org/frameworks/ki18n/html/prg_guide.html

It would be cool to have the doc link in the error message, but then the message would be even longer, and given the correct method call it's pretty straightforward to find those docs.