nixos/docs: document picom module changes

nixos/doc/manual/from_md/release-notes/rl-2211.section.xml

@@ -1464,6 +1464,14 @@ services.github-runner.serviceOverrides.SupplementaryGroups = [
           are under <literal>programs.firefox</literal>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The option
+          <literal>services.picom.experimentalBackends</literal> was
+          removed since it is now the default and the option will cause
+          <literal>picom</literal> to quit instead.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

nixos/doc/manual/release-notes/rl-2211.section.md

@@ -447,4 +447,6 @@ Available as [services.patroni](options.html#opt-services.patroni.enable).
 
 - A NixOS module for Firefox has been added which allows preferences and [policies](https://github.com/mozilla/policy-templates/blob/master/README.md) to be set. This also allows extensions to be installed via the `ExtensionSettings` policy. The new options are under `programs.firefox`.
 
+- The option `services.picom.experimentalBackends` was removed since it is now the default and the option will cause `picom` to quit instead.
+
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