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.slice.xml

@@ -97,6 +97,14 @@
     url="http://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface/">New
     Control Group Interfaces</ulink> for an introduction on how to make
     use of slice units from programs.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>Slice units automatically gain dependencies of type
+    <varname>After=</varname> and <varname>Requires=</varname> on
+    their immediate parent slice unit.</para>
 
     <para>Unless <varname>DefaultDependencies=false</varname>
     is used, slice units will implicitly have dependencies of