added marker lines

HOWTO.md

@@ -404,7 +404,9 @@ samples existed before the RRD was created.
 
 Temperature uses the left axis with a default range of 0–30°C. Humidity and
 battery use the right axis with a default range of 0–100%. These ranges and
-labels can be changed in `config.yaml` under `graph.axes`.
+labels can be changed in `config.yaml` under `graph.axes`. Light-grey reference
+lines are drawn at 15, 20, and 25°C by default and can be changed under
+`graph.axes.temperature.reference_lines`.
 
 The collector also generates `/var/www/html/sensors/index.html`. It displays
 six graphs at a time and has controls for switching between the daily, monthly,
@@ -442,9 +444,9 @@ To run it automatically as a user service, copy the supplied service file. It
 assumes this repository is located at `$HOME/service_zigbee`:
 
 ```bash
-cp zigbee-sensor.service.example /etc/systemd/system
-systemctl --user daemon-reload
-systemctl --user enable --now zigbee-sensor.service
+cp zigbee-sensors.service.example /etc/systemd/system
+systemctl daemon-reload
+systemctl enable --now zigbee-sensors.service
 ```
 
 Inspect its status and live logs with: