etc/linux-systemd: Mention AmbientCapabilities for syncOwnership. (#8536)

Add a commented entry to the systemd service file templates to point
the user in the right direction when using syncOwnership and starting
via systemd.  Which is more upgrade-friendly than setting caps on the
executable directly, as mentioned in the docs.

etc/linux-systemd/README.md

@@ -5,4 +5,4 @@ This directory contains configuration files for running Syncthing under the
 systemd user service. For further documentation take a look at the [systemd
 section][1] on https://docs.syncthing.net.
 
-[1]: https://docs.syncthing.net/users/autostart.html#using-systemd
+[1]: https://docs.syncthing.net/users/autostart#using-systemd

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

@@ -20,5 +20,9 @@ SystemCallArchitectures=native
 MemoryDenyWriteExecute=true
 NoNewPrivileges=true
 
+# Elevated permissions to sync ownership (disabled by default),
+# see https://docs.syncthing.net/advanced/folder-sync-ownership
+#AmbientCapabilities=CAP_CHOWN CAP_FOWNER
+
 [Install]
 WantedBy=multi-user.target

etc/linux-systemd/user/syncthing.service

@@ -16,5 +16,9 @@ SystemCallArchitectures=native
 MemoryDenyWriteExecute=true
 NoNewPrivileges=true
 
+# Elevated permissions to sync ownership (disabled by default),
+# see https://docs.syncthing.net/advanced/folder-sync-ownership
+#AmbientCapabilities=CAP_CHOWN CAP_FOWNER
+
 [Install]
 WantedBy=default.target