Make "don't run me with sudo/kdesu" message informative

We tell users what not to do, but we don't tell then what to do instead.
At one point we told them to just use Dolphin normally and the polkit
integration would kick in, but that work ended up being reverted and
abandoned. Instead, tell them to use kio-admin, which exists as a
released product and will hopefully not get removed anytime soon.

CCBUG: 467948