Add 'blocking move' comments

Marlin/src/module/motion.cpp

     planner.e_factor[active_extruder] = 1.0f;
   #endif
 
-  #if IS_KINEMATIC
-    if (is_fast)
-      prepare_fast_move_to_destination();
-    else
+  #if !IS_KINEMATIC
+    constexpr bool is_fast = false;
   #endif
-      prepare_line_to_destination();
+  if (is_fast)
+    prepare_fast_move_to_destination();
+  else
+    prepare_line_to_destination();
 
   feedrate_mm_s = old_feedrate;
   feedrate_percentage = old_pct;
 }
 
 /**
- * Plan a move to (X, Y, Z) and set the current_position
+ * Plan a move to (X, Y, Z) with separation of the XY and Z components.
+ *
+ * - If Z is moving up, the Z move is done before XY.
+ * - If Z is moving down, the Z move is done after XY.
+ * - Delta may lower Z first to get into the free motion zone.
+ * - Before returning, wait for the planner buffer to empty.
  */
 void do_blocking_move_to(const float rx, const float ry, const float rz, const_feedRate_t fr_mm_s/*=0.0*/) {
   DEBUG_SECTION(log_move, "do_blocking_move_to", DEBUGGING(LEVELING));
   const feedRate_t z_feedrate = fr_mm_s ?: homing_feedrate(Z_AXIS),
                   xy_feedrate = fr_mm_s ?: feedRate_t(XY_PROBE_FEEDRATE_MM_S);
 
-  #if ENABLED(DELTA)
-
+  #if EITHER(DELTA, IS_SCARA)
     if (!position_is_reachable(rx, ry)) return;
+    destination = current_position;          // sync destination at the start
+  #endif
 
-    REMEMBER(fr, feedrate_mm_s, xy_feedrate);
+  #if ENABLED(DELTA)
 
-    destination = current_position;          // sync destination at the start
+    REMEMBER(fr, feedrate_mm_s, xy_feedrate);
 
     if (DEBUGGING(LEVELING)) DEBUG_POS("destination = current_position", destination);
 
     // when in the danger zone
     if (current_position.z > delta_clip_start_height) {
-      if (rz > delta_clip_start_height) {   // staying in the danger zone
-        destination.set(rx, ry, rz);        // move directly (uninterpolated)
+      if (rz > delta_clip_start_height) {                     // staying in the danger zone
+        destination.set(rx, ry, rz);                          // move directly (uninterpolated)
         prepare_internal_fast_move_to_destination();          // set current_position from destination
         if (DEBUGGING(LEVELING)) DEBUG_POS("danger zone move", current_position);
         return;
 
   #elif IS_SCARA
 
-    if (!position_is_reachable(rx, ry)) return;
-
-    destination = current_position;
-
     // If Z needs to raise, do it before moving XY
     if (destination.z < rz) {
       destination.z = rz;