man: update the description of offline updates

- use "service" instead of "script", because various offline updaters that we have
  aren't really scripts, e.g. dnf-plugin-system-upgrade, packagekit-offline-update,
 fwupd-offline-update.
- strongly recommend After=sysinit.target, Wants=sysinit.target
- clarify a bit what should happen when multiple update services are started
- replace links to the wiki with refs to the man page that replaced it.

man/systemd.offline-updates.xml

@@ -86,34 +86,42 @@
       </listitem>
 
       <listitem>
-        <para>The system now continues to boot into <filename>default.target</filename>, and thus
-        into <filename>system-update.target</filename>. This target pulls in the system update unit,
-        which starts the system update script after all file systems have been mounted.</para>
+        <para>The system now continues to boot into <filename>default.target</filename>, and
+        thus into <filename>system-update.target</filename>. This target pulls in all system
+        update units. Only one service should perform an update (see the next point), and all
+        the other ones should exit cleanly with a "success" return code and without doing
+        anything. Update services should be ordered after <filename>sysinit.target</filename>
+        so that the update starts after after all file systems have been mounted.</para>
       </listitem>
 
       <listitem>
-        <para>As the first step, the update script should check if the
+        <para>As the first step, an update service should check if the
         <filename>/system-update</filename> symlink points to the location used by that update
-        script. In case it does not exists or points to a different location, the script must exit
+        service. In case it does not exist or points to a different location, the service must exit
         without error. It is possible for multiple update services to be installed, and for multiple
-        update scripts to be launched in parallel, and only the one that corresponds to the tool
+        update services to be launched in parallel, and only the one that corresponds to the tool
         that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
         is unsafe to run multiple updates in parallel.</para>
       </listitem>
 
       <listitem>
-        <para>The update script should now do its job. If applicable and possible, it should
-        create a file system snapshot, then install all packages.
-        After completion (regardless whether the update succeeded or failed) the machine
-        must be rebooted, for example by calling <command>systemctl reboot</command>.
-        In addition, on failure the script should revert to the old file system snapshot
-        (without the symlink).</para>
+        <para>The update service should now do its job. If applicable and possible, it should
+        create a file system snapshot, then install all packages.  After completion (regardless
+        whether the update succeeded or failed) the machine must be rebooted, for example by
+        calling <command>systemctl reboot</command>. In addition, on failure the script should
+        revert to the old file system snapshot (without the symlink).</para>
       </listitem>
 
       <listitem>
-        <para>The system is rebooted. Since the <filename>/system-update</filename> symlink is gone,
-        the generator won't redirect <filename>default.target</filename> after reboot and the
-        system now boots into the default target again.</para>
+        <para>The upgrade scripts should exit only after the update is finished. It is expected
+        that the service which performs the upgrade will cause the machine to reboot after it
+        is done.</para>
+      </listitem>
+
+      <listitem>
+        <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
+        the generator won't redirect <filename>default.target</filename> anymore and the system
+        now boots into the default target again.</para>
       </listitem>
     </orderedlist>
   </refsect1>
@@ -150,7 +158,8 @@
 
       <listitem>
         <para>The update service should declare <varname>DefaultDependencies=false</varname>,
-        and pull in any services it requires explicitly.</para>
+        <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
+        and explicitly pull in any other services it requires.</para>
       </listitem>
     </orderedlist>
   </refsect1>

man/systemd.special.xml

@@ -609,14 +609,12 @@
       <varlistentry>
         <term><filename>system-update.target</filename></term>
         <listitem>
-          <para>A special target unit that is used for off-line system
-          updates.
+          <para>A special target unit that is used for offline system updates.
           <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
-          will redirect the boot process to this target if
-          <filename>/system-update</filename> exists. For more
-          information see the <ulink
-          url="http://freedesktop.org/wiki/Software/systemd/SystemUpdates">System
-          Updates Specification</ulink>.</para>
+          will redirect the boot process to this target if <filename>/system-update</filename>
+          exists. For more information see
+          <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+          </para>
         </listitem>
       </varlistentry>
       <varlistentry>

units/system-update.target

@@ -6,8 +6,8 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=System Update
-Documentation=http://freedesktop.org/wiki/Software/systemd/SystemUpdates
+Description=Offline System Update
+Documentation=man:systemd.offline-updates(7)
 Documentation=man:systemd.special(7) man:systemd-system-update-generator(8)
 Requires=sysinit.target
 Conflicts=shutdown.target