You're reading the manual for Guix 1.4.0, which was released more than a few years ago! You might find the development version of the manual more helpful for documentation on plasma-desktop-service-type: https://guix.gnu.org/manual/devel/en/html_node/Desktop-Services.html#index-plasma_002ddesktop_002dservice_002dtype
I have Plasma configurations here, although it's been a while since I've used them (am on GNOME at the moment): https://github.com/aurtzy/guix-config/blob/master/modules/my-guix/mods/desktop-environment.scm#L110
Also, if you downloaded the 1.4.0 installer, you might have to guix pull (so the current state of Guix's plasma is made available) before trying to do any reconfiguring.