Add comments to debug bit flags

Marlin/Marlin.h

@@ -233,12 +233,12 @@ void kill(const char*);
  */
 enum DebugFlags {
   DEBUG_NONE          = 0,
-  DEBUG_ECHO          = _BV(0),
-  DEBUG_INFO          = _BV(1),
-  DEBUG_ERRORS        = _BV(2),
-  DEBUG_DRYRUN        = _BV(3),
-  DEBUG_COMMUNICATION = _BV(4),
-  DEBUG_LEVELING      = _BV(5)
+  DEBUG_ECHO          = _BV(0), ///< Echo commands in order as they are processed
+  DEBUG_INFO          = _BV(1), ///< Print messages for code that has debug output
+  DEBUG_ERRORS        = _BV(2), ///< Not implemented
+  DEBUG_DRYRUN        = _BV(3), ///< Ignore temperature setting and E movement commands
+  DEBUG_COMMUNICATION = _BV(4), ///< Not implemented
+  DEBUG_LEVELING      = _BV(5)  ///< Print detailed output for homing and leveling
 };
 extern uint8_t marlin_debug_flags;
 #define DEBUGGING(F) (marlin_debug_flags & (DEBUG_## F))