man: update description of exit.target

exit.target is now used for both system and user sessions,
so remove "on user service manager exit". Also reword that
paragraph: services will be killed before the manager exits,
even if they do not conflict with shutdown target, but we
recommend that they conflict with shutdown target so that
systemd schedules them to be stopped immediately when starting
to exit.

In the first paragraph, containers should be mentioned last,
and the more general systems first.

man/systemd.special.xml

@@ -215,20 +215,19 @@
         <term><filename>exit.target</filename></term>
         <listitem>
           <para>A special service unit for shutting down the system or
-          user service manager. It also works in containers and is
-          equivalent to <filename>poweroff.target</filename> on
-          non-container systems.</para>
+          user service manager. It is equivalent to
+          <filename>poweroff.target</filename> on non-container
+          systems, and also works in containers.</para>
 
-          <para>Applications wanting to terminate the user service
-          manager should start this unit. If systemd receives
+          <para>systemd will start this unit when it receives a
+          request to shut down over D-Bus or a
           <constant>SIGTERM</constant> or <constant>SIGINT</constant>
-          when running as user service daemon, it will start this
-          unit.</para>
+          signal when running as user service daemon.</para>
 
-          <para>Normally, this pulls in
+          <para>Normally, this (indirectly) pulls in
           <filename>shutdown.target</filename> which in turn should be
-          conflicted by all units that want to be shut down on user
-          service manager exit.</para>
+          conflicted by all units that want to be scheduled for
+          shutdown when the service manager starts to exit.</para>
         </listitem>
       </varlistentry>
       <varlistentry>