doc: Add an example for target units (#5951)

This adds an example target unit to the man page of systemd targets.

Closes #67.

man/systemd.target.xml

@@ -98,6 +98,43 @@
 
   </refsect1>
 
+  <refsect1>
+    <title>Example</title>
+
+    <example>
+      <title>Simple standalone target</title>
+
+      <programlisting># emergency-net.target
+
+[Unit]
+Description=Emergency Mode with Networking
+Requires=emergency.target systemd-networkd.service
+After=emergency.target systemd-networkd.service
+AllowIsolate=yes</programlisting>
+
+      <para>When adding dependencies to other units, it's important to check if they set
+      <varname>DefaultDependencies=</varname>. Service units, unless they set
+      <varname>DefaultDependencies=no</varname>, automatically get a dependency on
+      <filename>sysinit.target</filename>. In this case, both
+      <filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
+      have <varname>DefaultDependencies=no</varname>, so they are suitable for use
+      in this target, and do not pull in <filename>sysinit.target</filename>.</para>
+
+      <para>You can now switch into this emergency mode by running <varname>systemctl
+      isolate emergency-net.target</varname> or by passing the option
+      <varname>systemd.unit=emergency-net.target</varname> on the kernel command
+      line.</para>
+
+      <para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
+      <varname>[Install]</varname> section. After they are enabled using
+      <command>systemctl enable</command>, they will be started before
+      <varname>emergency-net.target</varname> is started. It is also possible to add
+      arbitrary units as dependencies of <filename>emergency.target</filename> without
+      modifying them by using <command>systemctl add-wants</command>.
+      </para>
+    </example>
+  </refsect1>
+
   <refsect1>
       <title>See Also</title>
       <para>