systemd: document the service files for split mode

pipewire/wireplumber#84 (comment 1117863)
7 jobs for wip/document-service-files in 3 minutes and 38 seconds (queued for 21 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #15620636

00:01:11

passed container_ubuntu #15620637

00:00:18

 
  Build
passed build_on_fedora_no_docs #15620639

00:01:31

passed build_on_fedora_with_docs #15620638

00:01:38

passed build_on_ubuntu_no_gir #15620641

00:01:03

passed build_on_ubuntu_with_gir #15620640

00:01:44

 
  Analysis
passed shellcheck #15620642

00:00:41