nixos: alsa: better docstring

nixos/modules/services/audio/alsa.nix

@@ -54,6 +54,11 @@ in
           description = ''
             Whether to enable volume and capture control with keyboard media keys.
 
+            You want to leave this disabled if you run a desktop environment
+            like KDE, Gnome, Xfce, etc, as those handle such things themselves.
+            You might want to enable this if you run a minimalistic desktop
+            environment or work from bare linux ttys/framebuffers.
+
             Enabling this will turn on <option>services.actkbd</option>.
           '';
         };