Better explain the watchdog "problem" and rename the config define so it explains that the feature belongs to the watchdog.

Marlin/Configuration_adv.h

@@ -185,9 +185,10 @@
 //#define USE_WATCHDOG
 
 #ifdef USE_WATCHDOG
-// you cannot watchdog reboot on Arduino mega2560 due to a bug in he bootloader. Hence we need to ask the user to reset.
+// If you have a watchdog reboot in an ArduinoMega2560 then the device will hang forever, as a watchdog reset will leave the watchdog on.
-//  THIS FEATURE IS UNSAFE!, as it will only work if interrupts are disabled. And the code could hang in an interrupt routine with interrupts disabled.
+// The "WATCHDOG_RESET_MANUAL" goes around this by not using the hardware reset.
-//#define RESET_MANUAL
+//  However, THIS FEATURE IS UNSAFE!, as it will only work if interrupts are disabled. And the code could hang in an interrupt routine with interrupts disabled.
+#define WATCHDOG_RESET_MANUAL
 #endif
 
 // extruder advance constant (s2/mm3)

Marlin/Marlin.h

@@ -17,7 +17,6 @@
 #include <util/delay.h>
 #include <avr/pgmspace.h>
 #include <avr/eeprom.h>
-#include <avr/wdt.h>
 #include <avr/interrupt.h>
 
 

Marlin/watchdog.cpp

@@ -1,6 +1,8 @@
 #include "Marlin.h"
 
 #ifdef USE_WATCHDOG
+#include <avr/wdt.h>
+
 #include "watchdog.h"
 #include "ultralcd.h"
 
@@ -16,7 +18,7 @@
 /// intialise watch dog with a 1 sec interrupt time
 void watchdog_init()
 {
-#ifdef RESET_MANUAL
+#ifdef WATCHDOG_RESET_MANUAL
     //We enable the watchdog timer, but only for the interrupt.
     //Take care, as this requires the correct order of operation, with interrupts disabled. See the datasheet of any AVR chip for details.
     wdt_reset();
@@ -30,7 +32,7 @@ void watchdog_init()
 /// reset watchdog. MUST be called every 1s after init or avr will reset.
 void watchdog_reset() 
 {
-  wdt_reset();
+    wdt_reset();
 }
 
 //===========================================================================
@@ -38,14 +40,14 @@ void watchdog_reset()
 //===========================================================================
 
 //Watchdog timer interrupt, called if main program blocks >1sec and manual reset is enabled.
-#ifdef RESET_MANUAL
+#ifdef WATCHDOG_RESET_MANUAL
 ISR(WDT_vect)
 { 
+    //TODO: This message gets overwritten by the kill() call
     LCD_MESSAGEPGM("ERR:Please Reset");//16 characters so it fits on a 16x2 display
     LCD_STATUS;
     SERIAL_ERROR_START;
     SERIAL_ERRORLNPGM("Something is wrong, please turn off the printer.");
-
     kill(); //kill blocks
     while(1); //wait for user or serial reset
 }