man: document automatic dependencies

For all units ensure there's an "Automatic Dependencies" section in the
man page, and explain which dependencies are automatically added in all
cases, and which ones are added on top if DefaultDependencies=yes is
set.

This is also done for systemd.exec(5), systemd.resource-control(5) and
systemd.unit(5) as these pages describe common behaviour of various unit
types.

man/systemd.timer.xml

@@ -1,4 +1,4 @@
-<?xml version='1.0'?> <!--*-nxml-*-->
+<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 
@@ -73,19 +73,29 @@
     <filename>foo.timer</filename> activates a matching service
     <filename>foo.service</filename>. The unit to activate may be
     controlled by <varname>Unit=</varname> (see below).</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>Timer units automatically gain a <varname>Before=</varname>
+    dependency on the service they are supposed to activate.</para>
 
     <para>Unless <varname>DefaultDependencies=</varname> is set to
     <option>false</option>, all timer units will implicitly have
-    dependencies of type <varname>Conflicts=</varname> and
-    <varname>Before=</varname> on <filename>shutdown.target</filename>
-    to ensure that they are stopped cleanly prior to system shutdown.
-    Timer units with at least one <varname>OnCalendar=</varname>
-    directive will have an additional <varname>After=</varname>
-    dependency on <filename>timer-sync.target</filename> to avoid
-    being started before the system clock has been correctly set. Only
-    timer units involved with early boot or late system shutdown
-    should disable the <varname>DefaultDependencies=</varname>
-    option.</para>
+    dependencies of type <varname>Requires=</varname> and
+    <varname>After=</varname> on <filename>sysinit.target</filename>,
+    a dependency of type <varname>Before=</varname> on
+    <filename>timers.target</filename>, as well as
+    <varname>Conflicts=</varname> and <varname>Before=</varname> on
+    <filename>shutdown.target</filename> to ensure that they are
+    stopped cleanly prior to system shutdown.  Timer units with at
+    least one <varname>OnCalendar=</varname> directive will have an
+    additional <varname>After=</varname> dependency on
+    <filename>timer-sync.target</filename> to avoid being started
+    before the system clock has been correctly set. Only timer units
+    involved with early boot or late system shutdown should disable
+    the <varname>DefaultDependencies=</varname> option.</para>
   </refsect1>
 
   <refsect1>