Switch missing default handler in OS detection docs (#26064)

Switch was missing default handler: control reaches end of non-void function

This warning gets treated as an error on Github Actions

docs/features/os_detection.md

@@ -136,6 +136,8 @@ bool process_record_user(uint16_t keycode, keyrecord_t *record) {
                 print_stored_setups();
             }
             return false;
+        default:
+            return true; // Process all other keycodes normally
     }
 }
 ```