Add comments for extruder offset options

Marlin/Marlin_main.cpp

 // Extruder offsets
 #if EXTRUDERS > 1
   #ifndef EXTRUDER_OFFSET_X
-    #define EXTRUDER_OFFSET_X { 0 }
+    #define EXTRUDER_OFFSET_X { 0 } // X offsets for each extruder
   #endif
   #ifndef EXTRUDER_OFFSET_Y
-    #define EXTRUDER_OFFSET_Y { 0 }
+    #define EXTRUDER_OFFSET_Y { 0 } // Y offsets for each extruder
   #endif
   float extruder_offset[][EXTRUDERS] = {
     EXTRUDER_OFFSET_X,
     EXTRUDER_OFFSET_Y
     #if ENABLED(DUAL_X_CARRIAGE)
-      , { 0 } // supports offsets in XYZ plane
+      , { 0 } // Z offsets for each extruder
     #endif
   };
 #endif
 #if EXTRUDERS > 1
 
   /**
-   * M218 - set hotend offset (in mm), T<extruder_number> X<offset_on_X> Y<offset_on_Y>
+   * M218 - set hotend offset (in mm)
+   *
+   *   T<tool>
+   *   X<xoffset>
+   *   Y<yoffset>
+   *   Z<zoffset> - Available with DUAL_X_CARRIAGE
    */
   inline void gcode_M218() {
     if (setTargetedHotend(218)) return;