Further cleanup of comments, partial Doxygen-style

Following up on #3231

Marlin/Configuration_adv.h

@@ -1,4 +1,4 @@
-/*
+/**
  * Marlin 3D Printer Firmware
  * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
  *
@@ -20,6 +20,16 @@
  *
  */
 
+/**
+ * Configuration_adv.h
+ *
+ * Advanced settings.
+ * Only change these if you know exactly what you're doing.
+ * Some of these settings can damage your printer if improperly set!
+ *
+ * Basic settings can be found in Configuration.h
+ *
+ */
 #ifndef CONFIGURATION_ADV_H
 #define CONFIGURATION_ADV_H