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.target.xml

@@ -77,15 +77,20 @@
     See
     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
     for details).</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Automatic Dependencies</title>
 
     <para>Unless <varname>DefaultDependencies=</varname> is set to
-    <option>false</option>, target units will implicitly complement
-    all configured dependencies of type <varname>Wants=</varname>,
+    <option>no</option>, target units will implicitly complement all
+    configured dependencies of type <varname>Wants=</varname>,
     <varname>Requires=</varname>,
     <varname>RequiresOverridable=</varname> with dependencies of type
-    <varname>After=</varname> if the units in question also have
-    <varname>DefaultDependencies=true</varname>.
-    </para>
+    <varname>After=</varname>, unless an ordering dependency of any
+    kind between the target and the respective other unit is already
+    in place. Note that this behaviour is disabled if either unit has
+    <varname>DefaultDependencies=no</varname>.</para>
   </refsect1>
 
   <refsect1>