nixos: Document option description changes

This adds a release notes entry to make users (and especially
developers) aware so they no longer need to use </para><para> in option
descriptions as this is now done automatically on every two consecutive
newlines.

More details can be found in the commit message of f865d0f.

Signed-off-by: aszlig <aszlig@nix.build>
(cherry picked from commit 4652f2f)

nixos/doc/manual/release-notes/rl-1809.xml

@@ -528,6 +528,14 @@ inherit (pkgs.nixos {
       to <literal>true</literal>.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      NixOS option descriptions are now automatically broken up into individual
+      paragraphs if the text contains two consecutive newlines, so it's no
+      longer necessary to use <code>&lt;/para&gt;&lt;para&gt;</code> to start
+      a new paragraph.
+    </para>
+  </listitem>
   </itemizedlist>
  </section>
 </section>