build: documet how to disable unit file instalation

meson_options.txt

@@ -1,2 +1,2 @@
 option('udev-dir', type: 'string', value: '', description: 'Absolute path of the udev base directory. Set to \'no\' not to install the udev rule')
-option('systemd-unit-dir', type: 'string', value: '', description: 'systemd service directory')
+option('systemd-unit-dir', type: 'string', value: '', description: 'Absolute path of the systemd service directory. Set to \'no\' not to install the radangeld service file.')