Clarify some motion code

Marlin/src/feature/bedlevel/abl/abl.cpp

 
     if (cx1 == cx2 && cy1 == cy2) {
       // Start and end on same mesh square
-      line_to_destination(fr_mm_s);
+      buffer_line_to_destination(fr_mm_s);
       set_current_from_destination();
       return;
     }
     }
     else {
       // Already split on a border
-      line_to_destination(fr_mm_s);
+      buffer_line_to_destination(fr_mm_s);
       set_current_from_destination();
       return;
     }

Marlin/src/feature/bedlevel/mbl/mesh_bed_leveling.cpp

 
     if (cx1 == cx2 && cy1 == cy2) {
       // Start and end on same mesh square
-      line_to_destination(fr_mm_s);
+      buffer_line_to_destination(fr_mm_s);
       set_current_from_destination();
       return;
     }
     }
     else {
       // Already split on a border
-      line_to_destination(fr_mm_s);
+      buffer_line_to_destination(fr_mm_s);
       set_current_from_destination();
       return;
     }

Marlin/src/feature/pause.cpp

   #if IS_KINEMATIC
     planner.buffer_line_kinematic(destination, fr, active_extruder);
   #else
-    line_to_destination(fr);
+    buffer_line_to_destination(fr);
   #endif
   stepper.synchronize();
 }

Marlin/src/gcode/bedlevel/mbl/G29.cpp

   #if ENABLED(MESH_G28_REST_ORIGIN)
     current_position[Z_AXIS] = Z_MIN_POS;
     set_destination_from_current();
-    line_to_destination(homing_feedrate(Z_AXIS));
+    buffer_line_to_destination(homing_feedrate(Z_AXIS));
     stepper.synchronize();
   #endif
 }

Marlin/src/inc/SanityCheck.h

    */
 
   #if ENABLED(DELTA)
-    #error "MESH_BED_LEVELING does not yet support DELTA printers."
+    #error "MESH_BED_LEVELING is not compatible with DELTA printers."
   #elif GRID_MAX_POINTS_X > 9 || GRID_MAX_POINTS_Y > 9
     #error "GRID_MAX_POINTS_X and GRID_MAX_POINTS_Y must be less than 10 for MBL."
   #endif

Marlin/src/module/motion.cpp

 /**
  * Cartesian Current Position
  *   Used to track the native machine position as moves are queued.
- *   Used by 'line_to_current_position' to do a move after changing it.
+ *   Used by 'buffer_line_to_current_position' to do a move after changing it.
  *   Used by 'SYNC_PLAN_POSITION_KINEMATIC' to update 'planner.position'.
  */
 float current_position[XYZE] = { 0.0 };
 
 /**
  * Cartesian Destination
- *   A temporary position, usually applied to 'current_position'.
- *   Set with 'get_destination_from_command' or 'set_destination_from_current'.
- *   'line_to_destination' sets 'current_position' to 'destination'.
+ *   The destination for a move, filled in by G-code movement commands,
+ *   and expected by functions like 'prepare_move_to_destination'.
+ *   Set with 'gcode_get_destination' or 'set_destination_from_current'.
  */
 float destination[XYZE] = { 0.0 };
 
  * Move the planner to the position stored in the destination array, which is
  * used by G0/G1/G2/G3/G5 and many other functions to set a destination.
  */
-void line_to_destination(const float fr_mm_s) {
+void buffer_line_to_destination(const float fr_mm_s) {
   planner.buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], fr_mm_s, active_extruder);
 }
 
       }
     #endif // HAS_MESH
 
-    line_to_destination(MMS_SCALED(feedrate_mm_s));
+    buffer_line_to_destination(MMS_SCALED(feedrate_mm_s));
     return false;
   }
 

Marlin/src/module/motion.h

  * Move the planner to the position stored in the destination array, which is
  * used by G0/G1/G2/G3/G5 and many other functions to set a destination.
  */
-void line_to_destination(const float fr_mm_s);
-
-inline void line_to_destination() { line_to_destination(feedrate_mm_s); }
+void buffer_line_to_destination(const float fr_mm_s);
 
 #if IS_KINEMATIC
   void prepare_uninterpolated_move_to_destination(const float fr_mm_s=0.0);

Marlin/src/module/planner.h

      *  fr_mm_s  - (target) speed of the move (mm/s)
      *  extruder - target extruder
      */
-    static FORCE_INLINE void buffer_line_kinematic(const float rtarget[XYZE], const float &fr_mm_s, const uint8_t extruder) {
+    static FORCE_INLINE void buffer_line_kinematic(const float cart[XYZE], const float &fr_mm_s, const uint8_t extruder) {
       #if PLANNER_LEVELING
-        float lpos[XYZ] = { rtarget[X_AXIS], rtarget[Y_AXIS], rtarget[Z_AXIS] };
-        apply_leveling(lpos);
+        float raw[XYZ] = { cart[X_AXIS], cart[Y_AXIS], cart[Z_AXIS] };
+        apply_leveling(raw);
       #else
-        const float * const lpos = rtarget;
+        const float * const raw = cart;
       #endif
       #if IS_KINEMATIC
-        inverse_kinematics(lpos);
-        _buffer_line(delta[A_AXIS], delta[B_AXIS], delta[C_AXIS], rtarget[E_AXIS], fr_mm_s, extruder);
+        inverse_kinematics(raw);
+        _buffer_line(delta[A_AXIS], delta[B_AXIS], delta[C_AXIS], cart[E_AXIS], fr_mm_s, extruder);
       #else
-        _buffer_line(lpos[X_AXIS], lpos[Y_AXIS], lpos[Z_AXIS], rtarget[E_AXIS], fr_mm_s, extruder);
+        _buffer_line(raw[X_AXIS], raw[Y_AXIS], raw[Z_AXIS], cart[E_AXIS], fr_mm_s, extruder);
       #endif
     }