Cleanup and dev notes in Marlin.h

Marlin/Marlin.h

 void FlushSerialRequestResend();
 void ok_to_send();
 
-#ifdef DELTA
-  void calculate_delta(float cartesian[3]);
-  #ifdef ENABLE_AUTO_BED_LEVELING
-    extern int delta_grid_spacing[2];
-    void adjust_delta(float cartesian[3]);
-  #endif
-  extern float delta[3];
-#endif
-#ifdef SCARA
-  void calculate_delta(float cartesian[3]);
-  void calculate_SCARA_forward_Transform(float f_scara[3]);
-#endif
 void reset_bed_level();
 void prepare_move();
 void kill(const char *);
 extern float filament_size[EXTRUDERS]; // cross-sectional area of filament (in millimeters), typically around 1.75 or 2.85, 0 disables the volumetric calculations for the extruder.
 extern float volumetric_multiplier[EXTRUDERS]; // reciprocal of cross-sectional area of filament (in square millimeters), stored this way to reduce computational burden in planner
 extern float current_position[NUM_AXIS];
-extern float home_offset[3];
-
-#ifdef DELTA
-  extern float endstop_adj[3];
-  extern float delta_radius;
-  extern float delta_diagonal_rod;
-  extern float delta_segments_per_second;
-  void recalc_delta_settings(float radius, float diagonal_rod);
-#elif defined(Z_DUAL_ENDSTOPS)
-  extern float z_endstop_adj;
-#endif
+extern float home_offset[3]; // axis[n].home_offset
+extern float min_pos[3]; // axis[n].min_pos
+extern float max_pos[3]; // axis[n].max_pos
+extern bool axis_known_position[3]; // axis[n].is_known
 
-#ifdef SCARA
-  extern float axis_scaling[3];  // Build size scaling
+#if defined(DELTA) || defined(SCARA)
+  extern float delta[3];
+  void calculate_delta(float cartesian[3]);
+  #ifdef DELTA
+    extern float endstop_adj[3]; // axis[n].endstop_adj
+    extern float delta_radius;
+    extern float delta_diagonal_rod;
+    extern float delta_segments_per_second;
+    void recalc_delta_settings(float radius, float diagonal_rod);
+    #ifdef ENABLE_AUTO_BED_LEVELING
+      extern int delta_grid_spacing[2];
+      void adjust_delta(float cartesian[3]);
+    #endif
+  #elif defined(SCARA)
+    extern float axis_scaling[3];  // Build size scaling
+    void calculate_SCARA_forward_Transform(float f_scara[3]);
+  #endif
 #endif
 
-extern float min_pos[3];
-extern float max_pos[3];
-extern bool axis_known_position[3];
+#ifdef Z_DUAL_ENDSTOPS
+  extern float z_endstop_adj;
+#endif
 
 #ifdef ENABLE_AUTO_BED_LEVELING
   extern float zprobe_zoffset;
 
 #ifdef FWRETRACT
   extern bool autoretract_enabled;
-  extern bool retracted[EXTRUDERS];
+  extern bool retracted[EXTRUDERS]; // extruder[n].retracted
   extern float retract_length, retract_length_swap, retract_feedrate, retract_zlift;
   extern float retract_recover_length, retract_recover_length_swap, retract_recover_feedrate;
 #endif