Add comment clarifying mango binary source in greetd config

Co-authored-by: ragusa-it <196988693+ragusa-it@users.noreply.github.com>

modules/common.nix

@@ -70,6 +70,7 @@
   services.seatd.enable = true;
 
   # Use greetd to automatically start a MangoWC session on login
+  # Note: 'mango' binary is provided by programs.mango.enable above
   services.greetd = {
     enable = true;
     settings.default_session = {