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.device.xml

@@ -83,7 +83,18 @@
     the escaping logic used to convert a file system path to a unit
     name see
     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+  </refsect1>
 
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>Many unit types automatically acquire dependencies on device
+    units of devices they require. For example,
+    <filename>.socket</filename> unit acquire dependencies on the
+    device units of the network interface specified in
+    <varname>BindToDevice=</varname>. Similar, swap and mount units
+    acquire dependencies on the units encapsulating their backing
+    block devices.</para>
   </refsect1>
 
   <refsect1>