Fix typo in MANUAL_HOME_POSITION (plural S missing) and improve comments.

Marlin/Configuration.h

 #define Z_MAX_LENGTH (Z_MAX_POS - Z_MIN_POS)
 
 // The position of the homing switches
-//#define MANUAL_HOME_POSITIONS  // If defined, manualy programed locations will be used
-//#define BED_CENTER_AT_0_0  // If defined the center of the bed is defined as (0,0)
+//#define MANUAL_HOME_POSITIONS  // If defined, MANUAL_*_HOME_POS below will be used
+//#define BED_CENTER_AT_0_0  // If defined, the center of the bed is at (X=0, Y=0)
 
 //Manual homing switch locations:
 #define MANUAL_X_HOME_POS 0

Marlin/Configuration_adv.h

 
 //// AUTOSET LOCATIONS OF LIMIT SWITCHES
 //// Added by ZetaPhoenix 09-15-2012
-#ifdef MANUAL_HOME_POSITION  //Use manual limit switch locations
+#ifdef MANUAL_HOME_POSITIONS  // Use manual limit switch locations
   #define X_HOME_POS MANUAL_X_HOME_POS
   #define Y_HOME_POS MANUAL_Y_HOME_POS
   #define Z_HOME_POS MANUAL_Z_HOME_POS