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.path.xml

@@ -79,13 +79,24 @@
     limitations as inotify, and for example cannot be used to monitor
     files or directories changed by other machines on remote NFS file
     systems.</para>
+  </refsect1>
 
-    <para>If a path unit is beneath another mount point in the file
-    system hierarchy, a dependency between both units is created
-    automatically.</para>
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>If a path unit is beneath another mount unit in the file
+    system hierarchy, both a requirement and an ordering dependency
+    between both units are created automatically.</para>
+
+    <para>An implicit <varname>Before=</varname> dependency is added
+    between a path unit and the unit it is supposed to activate.</para>
 
     <para>Unless <varname>DefaultDependencies=false</varname> is used,
     path units will implicitly have dependencies of type
+    <varname>Before=</varname> on <filename>paths.target</filename>,
+    dependencies of type <varname>After=</varname> and
+    <varname>Requires=</varname> on
+    <filename>sysinit.target</filename>, and have dependencies of type
     <varname>Conflicts=</varname> and <varname>Before=</varname> on
     <filename>shutdown.target</filename>. These ensure that path units
     are terminated cleanly prior to system shutdown. Only path units