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.resource-control.xml

@@ -89,6 +89,15 @@
     use of resource control APIs from programs.</para>
   </refsect1>
 
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>Units with the <varname>Slice=</varname> setting set get
+    automatic <varname>Requires=</varname> and
+    <varname>After=</varname> dependencies on the specified slice
+    unit.</para>
+  </refsect1>
+
   <refsect1>
     <title>Options</title>