man: expand description of lingering and KillUserProcesses setting

The description in the man page was wrong, KillUserProcesses does
not kill all processes of the user. Describe what the setting
does, and also add links between the relavant sections of the
manual.

Also, add an extensive example which shows how to launch screen
in the background.

1 files changed, 44 insertions, 0 deletions

diff --git a/man/systemd-run.xml b/man/systemd-run.xml
index 7b7c9305fb..0d58356e00 100644
--- a/man/systemd-run.xml
+++ b/man/systemd-run.xml
@@ -394,6 +394,50 @@ Dec 08 20:44:48 container systemd[1]: Started /bin/touch /tmp/foo.</programlisti
 
       <programlisting># systemd-run -t --send-sighup /bin/bash</programlisting>
     </example>
+
+    <example>
+      <title>Start <command>screen</command> as a user service</title>
+
+      <programlisting>$ systemd-run --scope --user screen
+Running scope as unit run-r14b0047ab6df45bfb45e7786cc839e76.scope.
+
+$ screen -ls
+There is a screen on:
+        492..laptop     (Detached)
+1 Socket in /var/run/screen/S-fatima.
+</programlisting>
+
+      <para>This starts the <command>screen</command> process as a child of the
+      <command>systemd --user</command> process that was started by
+      <filename>user@.service</filename>, in a scope unit. A
+      <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      unit is used instead of a
+      <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      unit, because <command>screen</command> will exit when detaching from the terminal,
+      and a service unit would be terminated. Running <command>screen</command>
+      as a user unit has the advantage that it is not part of the session scope.
+      If <varname>KillUserProcesses=yes</varname> is configured in
+      <citerefentry><refentrytitle>logind.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      the default, the session scope will be terminated when the user logs
+      out of that session.</para>
+
+      <para>The <filename>user@.service</filename> is started automatically
+      when the user first logs in, and stays around as long as at least one
+      login session is open. After the user logs out of the last session,
+      <filename>user@.service</filename> and all services underneath it
+      are terminated. This behaviour is the default, when "lingering" is
+      not enabled for that user. Enabling lingering means that
+      <filename>user@.service</filename> is started automatically during
+      boot, even if the user is not logged in, and that the service is
+      not terminated when the user logs out.</para>
+
+      <para>Enabling lingering allows the user to run processes without being logged in,
+      for example to allow <command>screen</command> to persist after the user logs out,
+      even if the session scope is terminated. In the default configuration, users can
+      enable lingering for themselves:</para>
+
+      <programlisting>$ loginctl enable-linger</programlisting>
+    </example>
   </refsect1>
 
   <refsect1>