Add a dbus-update-activation-environment invocation that users can
add to their config file. This should work for most users, assuming
Sway, systemd, and D-Bus activation.
The --systemd flag also imports the variables to the systemd user
session.
Add -r flag to xdpw to make sure any currently running process gets
replaced.
Remove the /usr/lib/xdg-desktop-portal invocation since xdpw can now be
replaced.
Adapt the wording to the latest xdpw changes.