Filament sensor changes to config file

Update the config file for improvements and clarifying what diameter to
use in the slicer software.

Marlin/Configuration.h

 #define FILAMENT_SENSOR_EXTRUDER_NUM	0  //The number of the extruder that has the filament sensor (0,1,2)
 #define MEASUREMENT_DELAY_CM			14  //measurement delay in cm.  This is the distance from filament sensor to middle of barrel
 
-#define DEFAULT_NOMINAL_FILAMENT_DIA  3.0  //Enter the diameter (in mm) of the filament generally used (3.0 mm or 1.75 mm).  Used for sensor reading validation
+#define DEFAULT_NOMINAL_FILAMENT_DIA  3.0  //Enter the diameter (in mm) of the filament generally used (3.0 mm or 1.75 mm) - this is then used in the slicer software.  Used for sensor reading validation
 #define MEASURED_UPPER_LIMIT          3.30  //upper limit factor used for sensor reading validation in mm
 #define MEASURED_LOWER_LIMIT          1.90  //lower limit factor for sensor reading validation in mm
 #define MAX_MEASUREMENT_DELAY			20  //delay buffer size in bytes (1 byte = 1cm)- limits maximum measurement delay allowable (must be larger than MEASUREMENT_DELAY_CM  and lower number saves RAM)
 
 //defines used in the code
 #define DEFAULT_MEASURED_FILAMENT_DIA  DEFAULT_NOMINAL_FILAMENT_DIA  //set measured to nominal initially 
-#define STANDARD_DIA					1.12837915  //The diameter of filament that has a cross sectional area of 1 square mm.  This dia should be used in the slicer software settings