docs: document labwc-session.target integration

Describe the shipped labwc-session.target in labwc(1) SESSION MANAGEMENT
and add commented-out systemctl start/stop lines to the example
autostart and shutdown files. Users on systemd-based distros can
uncomment these to pull in graphical-session.target when labwc starts
and tear it down cleanly on exit, without labwc itself mandating any
specific init system.

docs/autostart

@@ -1,5 +1,13 @@
 # Example autostart file
 
+# When running under systemd, uncomment the systemctl line below to pull in
+# graphical-session.target via labwc-session.target. This lets systemd user
+# services declaring WantedBy=graphical-session.target (panels, portals,
+# notification daemons, etc.) start in sync with the labwc session. Enable
+# individual services with: systemctl --user enable <unit>
+#
+# systemctl --user --no-block start labwc-session.target
+
 # Set background color.
 swaybg -c '#113344' >/dev/null 2>&1 &
 

docs/labwc.1.scd

@@ -118,6 +118,25 @@ this is accomplished by setting the session variables to empty strings. For
 systemd, the command `systemctl --user unset-environment` will be invoked to
 actually remove the variables from the activation environment.
 
+A systemd user unit named `labwc-session.target` is also shipped alongside
+the compositor for users who want to integrate labwc with systemd. It binds
+to the standard `graphical-session.target`, so systemd user services can
+start and stop in sync with the labwc session when they declare a WantedBy
+or PartOf relationship to that target. Labwc does not activate the target
+itself; users opt in by adding lines like the following to their
+*autostart* and *shutdown* files:
+
+```
+systemctl --user --no-block start labwc-session.target
+systemctl --user stop graphical-session.target
+```
+
+The example *autostart* and *shutdown* files shipped with labwc include
+these commented out. To have a user service automatically started with
+the session, enable it so the corresponding symlink under the
+graphical-session.target.wants directory exists, for example by running
+"systemctl --user enable dms.service".
+
 # ENVIRONMENT VARIABLES
 
 Set the environment variables listed below to enable specific debug options.

docs/shutdown

@@ -3,3 +3,11 @@
 # This file is executed as a shell script when labwc is preparing to terminate
 # itself.
 # For further details see labwc-config(5).
+
+# When running under systemd, uncomment the systemctl line below to tear down
+# graphical-session.target (which cascades to labwc-session.target via
+# BindsTo, and to any service declaring PartOf=graphical-session.target).
+# Running synchronously here ensures those services are stopped before the
+# Wayland socket goes away, avoiding "Broken pipe" failures on teardown.
+#
+# systemctl --user stop graphical-session.target