Add Documentation=... to sample systemd service files.

sample/systemd/borgmatic-user.service

@@ -3,6 +3,7 @@ Description=borgmatic backup
 Wants=network-online.target
 After=network-online.target
 ConditionACPower=true
+Documentation=https://torsion.org/borgmatic/
 
 [Service]
 Type=oneshot

sample/systemd/borgmatic.service

@@ -5,6 +5,7 @@ After=network-online.target
 # Prevent borgmatic from running unless the machine is plugged into power. Remove this line if you
 # want to allow borgmatic to run anytime.
 ConditionACPower=true
+Documentation=https://torsion.org/borgmatic/
 
 [Service]
 Type=oneshot