Let "systemctl help" command work

Before this change, issuing either
    systemctl --user help syncthing[.service]
or
    systemctl help syncthing@user[.service]
gave the message
    Can't show: http://docs.syncthing.net/

Following this change the syncthing man page is displayed

etc/linux-systemd/system/[email protected]

@@ -1,6 +1,6 @@
 [Unit]
 Description=Syncthing - Open Source Continuous File Synchronization for %I
-Documentation=http://docs.syncthing.net/
+Documentation=man:syncthing(1)
 After=network.target
 [email protected]
 

etc/linux-systemd/user/syncthing.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Syncthing - Open Source Continuous File Synchronization
-Documentation=http://docs.syncthing.net/
+Documentation=man:syncthing(1)
 After=network.target
 Wants=syncthing-inotify.service