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.unit.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" [
 <!ENTITY % entities SYSTEM "custom-entities.ent" >
@@ -195,12 +195,6 @@
          consider it mostly obsolete, and want people to
          use .d/ drop-ins instead. -->
 
-    <para>Note that while systemd offers a flexible dependency system
-    between units it is recommended to use this functionality only
-    sparingly and instead rely on techniques such as bus-based or
-    socket-based activation which make dependencies implicit,
-    resulting in a both simpler and more flexible system.</para>
-
     <para>Some unit names reflect paths existing in the file system
     namespace. Example: a device unit
     <filename>dev-sda.device</filename> refers to a device with the
@@ -253,6 +247,31 @@
 
   </refsect1>
 
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>Note that while systemd offers a flexible dependency system
+    between units it is recommended to use this functionality only
+    sparingly and instead rely on techniques such as bus-based or
+    socket-based activation which make dependencies implicit,
+    resulting in a both simpler and more flexible system.</para>
+
+    <para>A number of unit dependencies are automatically established,
+    depending on unit configuration. On top of that, for units with
+    <varname>DefaultDependencies=yes</varname> (the default) a couple
+    of additional dependencies are added. The precise effect of
+    <varname>DefaultDependencies=yes</varname> depends on the unit
+    type (see below).</para>
+
+    <para>If <varname>DefaultDependencies=yes</varname> is set, units
+    that are referenced by other units of type
+    <filename>.target</filename> via a <varname>Wants=</varname> or
+    <varname>Requires=</varname> dependency might automatically gain
+    an <varname>Before=</varname> dependency too. See
+    <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for details.</para>
+  </refsect1>
+
   <refsect1>
     <title>Unit File Load Path</title>