nixos/release-notes: Document dry activation scripts

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

@@ -37,6 +37,17 @@
           PostgreSQL now defaults to major version 13.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          Activation scripts can now opt int to be run when running
+          <literal>nixos-rebuild dry-activate</literal> and detect the
+          dry activation by reading <literal>$NIXOS_ACTION</literal>.
+          This allows activation scripts to output what they would
+          change if the activation was really run. The users/modules
+          activation script supports this and outputs some of is
+          actions.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-21.11-new-services">

nixos/doc/manual/release-notes/rl-2111.section.md

@@ -14,6 +14,10 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - PostgreSQL now defaults to major version 13.
 
+- Activation scripts can now opt int to be run when running `nixos-rebuild dry-activate` and detect the dry activation by reading `$NIXOS_ACTION`.
+  This allows activation scripts to output what they would change if the activation was really run.
+  The users/modules activation script supports this and outputs some of is actions.
+
 ## New Services {#sec-release-21.11-new-services}
 
 - [btrbk](https://digint.ch/btrbk/index.html), a backup tool for btrfs subvolumes, taking advantage of btrfs specific capabilities to create atomic snapshots and transfer them incrementally to your backup locations. Available as [services.btrbk](options.html#opt-services.brtbk.instances).