man: link Delegate= documentation up with the markdown docs

(cherry picked from commit 077c40b)

Related: #2215925

man/systemd.resource-control.xml

@@ -808,6 +808,9 @@
           specific to the unified hierarchy while others are specific to the legacy hierarchy. Also note that the
           kernel might support further controllers, which aren't covered here yet as delegation is either not supported
           at all for them or not defined cleanly.</para>
+
+          <para>For further details on the delegation model consult <ulink
+          url="https://systemd.io/CGROUP_DELEGATION">Control Group APIs and Delegation</ulink>.</para>
         </listitem>
       </varlistentry>