nixos/pulseaudio: disable flat-volumes by default

The motivation for this is that some applications are unaware
of this feature and can set their volume to 100% on startup
harming people ears and possiblly blowing someone's audio
setup.

I noticed this in #54594 and by extension epiphany[0].

Please also note that many other distros have this default for
the reason outlined above.

Closes #5632 #54594

[0]: https://bugzilla.gnome.org/show_bug.cgi?id=675217

nixos/doc/manual/release-notes/rl-1903.xml

@@ -423,6 +423,20 @@
       use <literal>nixos-rebuild boot; reboot</literal>.
      </para>
    </listitem>
+   <listitem>
+    <para>
+      Flat volumes are now disabled by default in <literal>hardware.pulseaudio</literal>.
+      This has been done to prevent applications, which are unaware of this feature, setting
+      their volumes to 100% on startup causing harm to your audio hardware and potentially your ears.
+    </para>
+    <note>
+     <para>
+      With this change application specific volumes are relative to the master volume which can be
+      adjusted independently, whereas before they were absolute; meaning that in effect, it scaled the
+      device-volume with the volume of the loudest application.
+     </para>
+    </note>
+   </listitem>
   </itemizedlist>
  </section>
 </section>