When using --replace arg with systemd, tell people about alternatives
Currently, when using the systemd boot process, running plasmashell --replace will still restart Plasma using KDBusAddons, rather than
restarting the systemd service for Plasma. This causes subtly different
codepaths to be taken, which can result in Plasma having different
behaviors when restarted using this method, compared to when Plasma is
started and restarted using systemd. What people should be doing is
running systemctl restart --user plasma-plasmashell.service instead.
However plasmashell --replace is a very old command, and the internet
is full of advice to use it, even though generally now they shouldn't.
But they don't know that!
To resolve that issue, this commit makes plasmashell --replace tell
the user what they should do instead in the future if they're not using
it for development or debugging purposes.
BUG: 474995 FIXED-IN: 6.0