man: mention pages with more settings at end of each option list

For some unit types we have hundreds of options, and the reader may easily miss
that more options are described in other pages. We already mentioned this in
the introduction and then at the top of the option list, but it can't hurt to
repeat the information.

Also, add an (almost empty) Options section for the unit types which don't have
any custom options. It is nicer to have the same page structure in all cases,
so people can jump between pages for different types more easily.

man/systemd.timer.xml

@@ -3,7 +3,7 @@
   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
 
-<refentry id="systemd.timer">
+<refentry id="systemd.timer" xmlns:xi="http://www.w3.org/2001/XInclude">
   <refentryinfo>
     <title>systemd.timer</title>
     <productname>systemd</productname>
@@ -94,7 +94,11 @@
   <refsect1>
     <title>Options</title>
 
-    <para>Timer files must include a [Timer] section, which carries
+    <para>Timer unit files may include [Unit] and [Install] sections, which are described in
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+    </para>
+
+    <para>Timer unit files must include a [Timer] section, which carries
     information about the timer it defines. The options specific to
     the [Timer] section of timer units are the following:</para>
 
@@ -178,7 +182,6 @@
         precise time configured with these settings, as they are
         subject to the <varname>AccuracySec=</varname> setting
         below.</para></listitem>
-
       </varlistentry>
 
       <varlistentry>
@@ -357,6 +360,8 @@
         <option>true</option>.</para></listitem>
       </varlistentry>
     </variablelist>
+
+    <xi:include href="systemd.service.xml" xpointer="shared-unit-options" />
   </refsect1>
 
   <refsect1>